Links¶
A knot is defined as embedding of the circle \(\mathbb{S}^1\) in the 3-dimensional sphere \(\mathbb{S}^3\), considered up to ambient isotopy. They represent the physical idea of a knotted rope, but with the particularity that the rope is closed. That is, the ends of the rope are joined.
A link is an embedding of one or more copies of \(\mathbb{S}^1\) in \(\mathbb{S}^3\), considered up to ambient isotopy. That is, a link represents the idea of one or more tied ropes. Every knot is a link, but not every link is a knot.
Generically, the projection of a link on \(\RR^2\) is a curve with crossings. The crossings are represented to show which strand goes over the other. This curve is called a planar diagram of the link. If we remove the crossings, the resulting connected components are segments. These segments are called the edges of the diagram.
REFERENCES:
See also
There are also tables of link and knot invariants at web-pages
KnotInfo and
LinkInfo. These can be
used inside Sage after installing the optional package
database_knotinfo (type sage -i database_knotinfo in a command shell,
see knotinfo).
AUTHORS:
Miguel Angel Marco Buzunariz
Amit Jamadagni
Sebastian Oehms (October 2020, add
get_knotinfo()andis_isotopic())Sebastian Oehms (May 2022): add
links_gould_polynomial()Sebastian Oehms (May 2023): change the convention about the
pd_codefrom clockwise to anti-clockwise (see Issue #35665).
- class sage.knots.link.Link(data)[source]¶
Bases:
SageObjectA link.
A link is an embedding of one or more copies of \(\mathbb{S}^1\) in \(\mathbb{S}^3\), considered up to ambient isotopy. That is, a link represents the idea of one or more tied ropes. Every knot is a link, but not every link is a knot.
A link can be created by using one of the conventions mentioned below:
Braid:
The closure of a braid is a link:
sage: B = BraidGroup(8) sage: L = Link(B([-1, -1, -1, -2, 1, -2, 3, -2, 3])); L Link with 1 component represented by 9 crossings sage: L = Link(B([1, 2, 1, -2, -1])); L Link with 2 components represented by 5 crossings
>>> from sage.all import * >>> B = BraidGroup(Integer(8)) >>> L = Link(B([-Integer(1), -Integer(1), -Integer(1), -Integer(2), Integer(1), -Integer(2), Integer(3), -Integer(2), Integer(3)])); L Link with 1 component represented by 9 crossings >>> L = Link(B([Integer(1), Integer(2), Integer(1), -Integer(2), -Integer(1)])); L Link with 2 components represented by 5 crossings
Note
The strands of the braid that have no crossings at all are removed.
Oriented Gauss Code:
Label the crossings from \(1\) to \(n\) (where \(n\) is the number of crossings) and start moving along the link. Trace every component of the link, by starting at a particular point on one component of the link and writing down each of the crossings that you encounter until returning to the starting point. The crossings are written with sign depending on whether we cross them as over or undercrossing. Each component is then represented as a list whose elements are the crossing numbers. A second list of \(+1\) and \(-1\)’s keeps track of the orientation of each crossing:
sage: L = Link([[[-1, 2, 3, -4, 5, -6, 7, 8, -2, -5, 6, 1, -8, -3, 4, -7]], ....: [-1, -1, -1, -1, 1, 1, -1, 1]]) sage: L Link with 1 component represented by 8 crossings
>>> from sage.all import * >>> L = Link([[[-Integer(1), Integer(2), Integer(3), -Integer(4), Integer(5), -Integer(6), Integer(7), Integer(8), -Integer(2), -Integer(5), Integer(6), Integer(1), -Integer(8), -Integer(3), Integer(4), -Integer(7)]], ... [-Integer(1), -Integer(1), -Integer(1), -Integer(1), Integer(1), Integer(1), -Integer(1), Integer(1)]]) >>> L Link with 1 component represented by 8 crossings
For links there may be more than one component and the input is as follows:
sage: L = Link([[[-1, 2], [-3, 4], [1, 3, -4, -2]], [-1, -1, 1, 1]]) sage: L Link with 3 components represented by 4 crossings
>>> from sage.all import * >>> L = Link([[[-Integer(1), Integer(2)], [-Integer(3), Integer(4)], [Integer(1), Integer(3), -Integer(4), -Integer(2)]], [-Integer(1), -Integer(1), Integer(1), Integer(1)]]) >>> L Link with 3 components represented by 4 crossings
Planar Diagram (PD) Code:
The diagram of the link is formed by segments that are adjacent to the crossings. Label each one of this segments with a positive number, and for each crossing, write down the four incident segments. The order of these segments is anti-clockwise, starting with the incoming undercrossing.
There is no particular distinction between knots and links for this input.
EXAMPLES:
One of the representations of the trefoil knot:
sage: L = Link([[1, 5, 2, 4], [5, 3, 6, 2], [3, 1, 4, 6]]) sage: L Link with 1 component represented by 3 crossings
>>> from sage.all import * >>> L = Link([[Integer(1), Integer(5), Integer(2), Integer(4)], [Integer(5), Integer(3), Integer(6), Integer(2)], [Integer(3), Integer(1), Integer(4), Integer(6)]]) >>> L Link with 1 component represented by 3 crossings
One of the representations of the Hopf link:
sage: L = Link([[1, 4, 2, 3], [4, 1, 3, 2]]) sage: L Link with 2 components represented by 2 crossings
>>> from sage.all import * >>> L = Link([[Integer(1), Integer(4), Integer(2), Integer(3)], [Integer(4), Integer(1), Integer(3), Integer(2)]]) >>> L Link with 2 components represented by 2 crossings
We can construct links from the braid group:
sage: B = BraidGroup(4) sage: L = Link(B([-1, -1, -1, -2, 1, -2, 3, -2])); L Link with 2 components represented by 8 crossings
>>> from sage.all import * >>> B = BraidGroup(Integer(4)) >>> L = Link(B([-Integer(1), -Integer(1), -Integer(1), -Integer(2), Integer(1), -Integer(2), Integer(3), -Integer(2)])); L Link with 2 components represented by 8 crossings
sage: L = Link(B([1, 2, 1, 3])); L Link with 2 components represented by 4 crossings
>>> from sage.all import * >>> L = Link(B([Integer(1), Integer(2), Integer(1), Integer(3)])); L Link with 2 components represented by 4 crossings
We construct the “monster” unknot using a planar code, and then construct the oriented Gauss code and braid representation:
sage: L = Link([[3,4,2,1], [8,7,1,9], [5,3,7,6], [4,5,6,18], ....: [17,18,8,19], [9,14,11,10], [10,11,13,12], ....: [12,13,15,19], [20,15,14,16], [16,2,17,20]]) sage: L.oriented_gauss_code() [[[1, -4, 3, -1, 10, -9, 6, -7, 8, 5, 4, -3, 2, -6, 7, -8, 9, -10, -5, -2]], [1, -1, 1, 1, 1, -1, -1, -1, -1, -1]] sage: L.braid() s0*s1^-3*s2^-1*s1*s3*s2^2*s1^-1*s0^-1*s2*s1^-1*s3^-1*s2*s1^-1
>>> from sage.all import * >>> L = Link([[Integer(3),Integer(4),Integer(2),Integer(1)], [Integer(8),Integer(7),Integer(1),Integer(9)], [Integer(5),Integer(3),Integer(7),Integer(6)], [Integer(4),Integer(5),Integer(6),Integer(18)], ... [Integer(17),Integer(18),Integer(8),Integer(19)], [Integer(9),Integer(14),Integer(11),Integer(10)], [Integer(10),Integer(11),Integer(13),Integer(12)], ... [Integer(12),Integer(13),Integer(15),Integer(19)], [Integer(20),Integer(15),Integer(14),Integer(16)], [Integer(16),Integer(2),Integer(17),Integer(20)]]) >>> L.oriented_gauss_code() [[[1, -4, 3, -1, 10, -9, 6, -7, 8, 5, 4, -3, 2, -6, 7, -8, 9, -10, -5, -2]], [1, -1, 1, 1, 1, -1, -1, -1, -1, -1]] >>> L.braid() s0*s1^-3*s2^-1*s1*s3*s2^2*s1^-1*s0^-1*s2*s1^-1*s3^-1*s2*s1^-1
We construct the Ochiai unknot by using an oriented Gauss code:
sage: L = Link([[[1,-2,-3,-8,-12,13,-14,15,-7,-1,2,-4,10,11,-13,12, ....: -11,-16,4,3,-5,6,-9,7,-15,14,16,-10,8,9,-6,5]], ....: [-1,-1,1,1,1,1,-1,1,1,-1,1,-1,-1,-1,-1,-1]]) sage: L.pd_code() [[10, 1, 11, 2], [2, 11, 3, 12], [3, 21, 4, 20], [12, 20, 13, 19], [21, 1, 22, 32], [31, 23, 32, 22], [9, 24, 10, 25], [4, 30, 5, 29], [23, 31, 24, 30], [28, 13, 29, 14], [17, 15, 18, 14], [5, 16, 6, 17], [15, 6, 16, 7], [7, 26, 8, 27], [25, 8, 26, 9], [18, 27, 19, 28]]
>>> from sage.all import * >>> L = Link([[[Integer(1),-Integer(2),-Integer(3),-Integer(8),-Integer(12),Integer(13),-Integer(14),Integer(15),-Integer(7),-Integer(1),Integer(2),-Integer(4),Integer(10),Integer(11),-Integer(13),Integer(12), ... -Integer(11),-Integer(16),Integer(4),Integer(3),-Integer(5),Integer(6),-Integer(9),Integer(7),-Integer(15),Integer(14),Integer(16),-Integer(10),Integer(8),Integer(9),-Integer(6),Integer(5)]], ... [-Integer(1),-Integer(1),Integer(1),Integer(1),Integer(1),Integer(1),-Integer(1),Integer(1),Integer(1),-Integer(1),Integer(1),-Integer(1),-Integer(1),-Integer(1),-Integer(1),-Integer(1)]]) >>> L.pd_code() [[10, 1, 11, 2], [2, 11, 3, 12], [3, 21, 4, 20], [12, 20, 13, 19], [21, 1, 22, 32], [31, 23, 32, 22], [9, 24, 10, 25], [4, 30, 5, 29], [23, 31, 24, 30], [28, 13, 29, 14], [17, 15, 18, 14], [5, 16, 6, 17], [15, 6, 16, 7], [7, 26, 8, 27], [25, 8, 26, 9], [18, 27, 19, 28]]
We construct the knot \(7_1\) and compute some invariants:
sage: B = BraidGroup(2) sage: L = Link(B([1]*7))
>>> from sage.all import * >>> B = BraidGroup(Integer(2)) >>> L = Link(B([Integer(1)]*Integer(7)))
sage: L.alexander_polynomial() t^-3 - t^-2 + t^-1 - 1 + t - t^2 + t^3 sage: L.jones_polynomial() -t^10 + t^9 - t^8 + t^7 - t^6 + t^5 + t^3 sage: L.determinant() 7 sage: L.signature() -6
>>> from sage.all import * >>> L.alexander_polynomial() t^-3 - t^-2 + t^-1 - 1 + t - t^2 + t^3 >>> L.jones_polynomial() -t^10 + t^9 - t^8 + t^7 - t^6 + t^5 + t^3 >>> L.determinant() 7 >>> L.signature() -6
The links here have removed components in which no strand is used:
sage: B = BraidGroup(8) sage: b = B([1]) sage: L = Link(b) sage: b.components_in_closure() 7 sage: L.number_of_components() 1 sage: L.braid().components_in_closure() 1 sage: L.braid().parent() Braid group on 2 strands
>>> from sage.all import * >>> B = BraidGroup(Integer(8)) >>> b = B([Integer(1)]) >>> L = Link(b) >>> b.components_in_closure() 7 >>> L.number_of_components() 1 >>> L.braid().components_in_closure() 1 >>> L.braid().parent() Braid group on 2 strands
Warning
Equality of knots is done by comparing the corresponding braids, which may give false negatives.
Note
The behavior of removing unused strands from an element of a braid group may change without notice in the future. Do not rely on this feature.
Todo
Implement methods to creating new links from previously created links.
- alexander_polynomial(var='t')[source]¶
Return the Alexander polynomial of
self.INPUT:
var– (default:'t') the variable in the polynomial
EXAMPLES:
We begin by computing the Alexander polynomial for the figure-eight knot:
sage: # needs sage.modules sage: B = BraidGroup(3) sage: L = Link(B([1, -2, 1, -2])) sage: L.alexander_polynomial() -t^-1 + 3 - t
>>> from sage.all import * >>> # needs sage.modules >>> B = BraidGroup(Integer(3)) >>> L = Link(B([Integer(1), -Integer(2), Integer(1), -Integer(2)])) >>> L.alexander_polynomial() -t^-1 + 3 - t
The “monster” unknot:
sage: L = Link([[3,1,2,4],[8,9,1,7],[5,6,7,3],[4,18,6,5], ....: [17,19,8,18],[9,10,11,14],[10,12,13,11], ....: [12,19,15,13],[20,16,14,15],[16,20,17,2]]) sage: L.alexander_polynomial() # needs sage.modules 1
>>> from sage.all import * >>> L = Link([[Integer(3),Integer(1),Integer(2),Integer(4)],[Integer(8),Integer(9),Integer(1),Integer(7)],[Integer(5),Integer(6),Integer(7),Integer(3)],[Integer(4),Integer(18),Integer(6),Integer(5)], ... [Integer(17),Integer(19),Integer(8),Integer(18)],[Integer(9),Integer(10),Integer(11),Integer(14)],[Integer(10),Integer(12),Integer(13),Integer(11)], ... [Integer(12),Integer(19),Integer(15),Integer(13)],[Integer(20),Integer(16),Integer(14),Integer(15)],[Integer(16),Integer(20),Integer(17),Integer(2)]]) >>> L.alexander_polynomial() # needs sage.modules 1
Some additional examples:
sage: # needs sage.modules sage: B = BraidGroup(2) sage: L = Link(B([1])) sage: L.alexander_polynomial() 1 sage: L = Link(B.one()) sage: L.alexander_polynomial() 1 sage: B = BraidGroup(3) sage: L = Link(B([1, 2, 1, 2])) sage: L.alexander_polynomial() t^-1 - 1 + t
>>> from sage.all import * >>> # needs sage.modules >>> B = BraidGroup(Integer(2)) >>> L = Link(B([Integer(1)])) >>> L.alexander_polynomial() 1 >>> L = Link(B.one()) >>> L.alexander_polynomial() 1 >>> B = BraidGroup(Integer(3)) >>> L = Link(B([Integer(1), Integer(2), Integer(1), Integer(2)])) >>> L.alexander_polynomial() t^-1 - 1 + t
When the Seifert surface is disconnected, the Alexander polynomial is defined to be \(0\):
sage: # needs sage.modules sage: B = BraidGroup(4) sage: L = Link(B([1,3])) sage: L.alexander_polynomial() 0
>>> from sage.all import * >>> # needs sage.modules >>> B = BraidGroup(Integer(4)) >>> L = Link(B([Integer(1),Integer(3)])) >>> L.alexander_polynomial() 0
See also
- arcs(presentation='pd')[source]¶
Return the arcs of
self.Arcs are the connected components of the planar diagram.
INPUT:
presentation– one of the following:'pd'– the arcs are returned as lists of parts in the PD code'gauss_code'– the arcs are returned as pieces of the Gauss code that start with a negative number, and end with the following negative one; of there exist a closed arc, it is returned as a list of positive numbers only
OUTPUT: list of lists representing the arcs based upon
presentationEXAMPLES:
sage: K = Knot([[[1,-2,3,-1,2,-3]],[1,1,1]]) sage: K.arcs() [[1, 2], [3, 4], [5, 6]] sage: K.arcs(presentation='gauss_code') [[-3, 1, -2], [-2, 3, -1], [-1, 2, -3]]
>>> from sage.all import * >>> K = Knot([[[Integer(1),-Integer(2),Integer(3),-Integer(1),Integer(2),-Integer(3)]],[Integer(1),Integer(1),Integer(1)]]) >>> K.arcs() [[1, 2], [3, 4], [5, 6]] >>> K.arcs(presentation='gauss_code') [[-3, 1, -2], [-2, 3, -1], [-1, 2, -3]]
sage: L = Link([[1, 2, 3, 4], [3, 2, 1, 4]]) sage: L.arcs() [[2, 4], [1], [3]] sage: L.arcs(presentation='gauss_code') [[-2, -1], [-1, -2], [2, 1]] sage: L.gauss_code() [[-1, -2], [2, 1]]
>>> from sage.all import * >>> L = Link([[Integer(1), Integer(2), Integer(3), Integer(4)], [Integer(3), Integer(2), Integer(1), Integer(4)]]) >>> L.arcs() [[2, 4], [1], [3]] >>> L.arcs(presentation='gauss_code') [[-2, -1], [-1, -2], [2, 1]] >>> L.gauss_code() [[-1, -2], [2, 1]]
- braid(remove_loops=False)[source]¶
Return a braid representation of
self.INPUT:
remove_loops– boolean (default:False); if set toTrueloops will be removed first. This can reduce the number of strands needed for an ambient isotopic braid closure. However, this can lead to a loss of the regular isotopy.
OUTPUT: an element in the braid group
Warning
For the unknot with no crossings, this returns the identity of the braid group with 2 strands because this disregards strands with no crossings.
EXAMPLES:
sage: L = Link([[2, 4, 1, 3], [4, 2, 3, 1]]) sage: L.braid() s^2 sage: L = Link([[[-1, 2, -3, 1, -2, 3]], [-1, -1, -1]]) sage: L.braid() s^-3 sage: L = Link([[1,7,2,8], [8,5,9,4], [3,10,4,9], [10,6,7,1], [5,2,6,3]]) sage: L.braid() (s0*s1^-1)^2*s1^-1
>>> from sage.all import * >>> L = Link([[Integer(2), Integer(4), Integer(1), Integer(3)], [Integer(4), Integer(2), Integer(3), Integer(1)]]) >>> L.braid() s^2 >>> L = Link([[[-Integer(1), Integer(2), -Integer(3), Integer(1), -Integer(2), Integer(3)]], [-Integer(1), -Integer(1), -Integer(1)]]) >>> L.braid() s^-3 >>> L = Link([[Integer(1),Integer(7),Integer(2),Integer(8)], [Integer(8),Integer(5),Integer(9),Integer(4)], [Integer(3),Integer(10),Integer(4),Integer(9)], [Integer(10),Integer(6),Integer(7),Integer(1)], [Integer(5),Integer(2),Integer(6),Integer(3)]]) >>> L.braid() (s0*s1^-1)^2*s1^-1
using
remove_loops=True:sage: L = Link([[2, 7, 1, 1], [7, 3, 9, 2], [4, 11, 3, 9], [11, 5, 5, 4]]) sage: L.braid() s0*s1^-1*s2*s3^-1 sage: L.braid(remove_loops=True) 1
>>> from sage.all import * >>> L = Link([[Integer(2), Integer(7), Integer(1), Integer(1)], [Integer(7), Integer(3), Integer(9), Integer(2)], [Integer(4), Integer(11), Integer(3), Integer(9)], [Integer(11), Integer(5), Integer(5), Integer(4)]]) >>> L.braid() s0*s1^-1*s2*s3^-1 >>> L.braid(remove_loops=True) 1
- coloring_maps(n=None, finitely_presented=False)[source]¶
Return the \(n\)-coloring maps of
self. These are group homomorphisms from the fundamental group ofselfto the \(n\)-th dihedral group.INPUT:
n– the number of colors to consider (if ommitted the value of the determinant ofselfwill be taken). Note that there are no coloring maps if n is coprime to the determinant ofselffinitely_presented– boolean (default:False); whether to choose the dihedral groups as finitely presented groups. If not set toTruethey are represented as permutation groups.
OUTPUT:
a list of group homomporhisms from the fundamental group of
selfto the \(n\)-th dihedral group (represented according to the key argumentfinitely_presented).EXAMPLES:
sage: L5a1_1 = Link([[8, 2, 9, 1], [10, 7, 5, 8], [4, 10, 1, 9], ....: [2, 5, 3, 6], [6, 3, 7, 4]]) sage: L5a1_1.determinant() 8 sage: L5a1_1.coloring_maps(2) [Group morphism: From: Finitely presented group < x0, x1, x2, x3, x4 | x4*x1*x0^-1*x1^-1, x0*x4^-1*x3^-1*x4, x2*x0*x1^-1*x0^-1, x1*x3^-1*x2^-1*x3, x3*x2^-1*x4^-1*x2 > To: Dihedral group of order 4 as a permutation group, Group morphism: From: Finitely presented group < x0, x1, x2, x3, x4 | x4*x1*x0^-1*x1^-1, x0*x4^-1*x3^-1*x4, x2*x0*x1^-1*x0^-1, x1*x3^-1*x2^-1*x3, x3*x2^-1*x4^-1*x2 > To: Dihedral group of order 4 as a permutation group] sage: col_maps = L5a1_1.coloring_maps(4); len(col_maps) 12 sage: col_maps = L5a1_1.coloring_maps(5); len(col_maps) 0 sage: col_maps = L5a1_1.coloring_maps(12); len(col_maps) 36 sage: col_maps = L5a1_1.coloring_maps(); len(col_maps) 56
>>> from sage.all import * >>> L5a1_1 = Link([[Integer(8), Integer(2), Integer(9), Integer(1)], [Integer(10), Integer(7), Integer(5), Integer(8)], [Integer(4), Integer(10), Integer(1), Integer(9)], ... [Integer(2), Integer(5), Integer(3), Integer(6)], [Integer(6), Integer(3), Integer(7), Integer(4)]]) >>> L5a1_1.determinant() 8 >>> L5a1_1.coloring_maps(Integer(2)) [Group morphism: From: Finitely presented group < x0, x1, x2, x3, x4 | x4*x1*x0^-1*x1^-1, x0*x4^-1*x3^-1*x4, x2*x0*x1^-1*x0^-1, x1*x3^-1*x2^-1*x3, x3*x2^-1*x4^-1*x2 > To: Dihedral group of order 4 as a permutation group, Group morphism: From: Finitely presented group < x0, x1, x2, x3, x4 | x4*x1*x0^-1*x1^-1, x0*x4^-1*x3^-1*x4, x2*x0*x1^-1*x0^-1, x1*x3^-1*x2^-1*x3, x3*x2^-1*x4^-1*x2 > To: Dihedral group of order 4 as a permutation group] >>> col_maps = L5a1_1.coloring_maps(Integer(4)); len(col_maps) 12 >>> col_maps = L5a1_1.coloring_maps(Integer(5)); len(col_maps) 0 >>> col_maps = L5a1_1.coloring_maps(Integer(12)); len(col_maps) 36 >>> col_maps = L5a1_1.coloring_maps(); len(col_maps) 56
applying the map:
sage: cm1 = col_maps[0] sage: gs = L5a1_1.fundamental_group().gens() sage: d = cm1(gs[0]); d (1,8)(2,7)(3,6)(4,5) sage: d.parent() Dihedral group of order 16 as a permutation group
>>> from sage.all import * >>> cm1 = col_maps[Integer(0)] >>> gs = L5a1_1.fundamental_group().gens() >>> d = cm1(gs[Integer(0)]); d (1,8)(2,7)(3,6)(4,5) >>> d.parent() Dihedral group of order 16 as a permutation group
using the finitely presented dihedral group:
sage: col_maps = L5a1_1.coloring_maps(2, finitely_presented=True) sage: d = col_maps[0](gs[1]); d b*a sage: d.parent() Finitely presented group < a, b | a^2, b^2, (a*b)^2 >
>>> from sage.all import * >>> col_maps = L5a1_1.coloring_maps(Integer(2), finitely_presented=True) >>> d = col_maps[Integer(0)](gs[Integer(1)]); d b*a >>> d.parent() Finitely presented group < a, b | a^2, b^2, (a*b)^2 >
REFERENCES:
Chapter 3 of [Liv1993]
See also
- colorings(n=None)[source]¶
Return the
n-colorings ofself.INPUT:
n– the number of colors to consider (if ommitted the value of the determinant ofselfwill be taken). Note that there are no colorings if n is coprime to the determinant ofself
OUTPUT:
a list with the colorings. Each coloring is represented as a dictionary that maps a tuple of the edges forming each arc (as in the PD code) to the index of the corresponding color.
EXAMPLES:
sage: K = Link([[[1, -2, 3, -1, 2, -3]], [1, 1, 1]]) sage: K.colorings(3) # needs sage.libs.pari sage.modules [{(1, 2): 0, (3, 4): 1, (5, 6): 2}, {(1, 2): 0, (3, 4): 2, (5, 6): 1}, {(1, 2): 1, (3, 4): 0, (5, 6): 2}, {(1, 2): 1, (3, 4): 2, (5, 6): 0}, {(1, 2): 2, (3, 4): 0, (5, 6): 1}, {(1, 2): 2, (3, 4): 1, (5, 6): 0}] sage: K.pd_code() [[4, 2, 5, 1], [2, 6, 3, 5], [6, 4, 1, 3]] sage: K.arcs('pd') [[1, 2], [3, 4], [5, 6]]
>>> from sage.all import * >>> K = Link([[[Integer(1), -Integer(2), Integer(3), -Integer(1), Integer(2), -Integer(3)]], [Integer(1), Integer(1), Integer(1)]]) >>> K.colorings(Integer(3)) # needs sage.libs.pari sage.modules [{(1, 2): 0, (3, 4): 1, (5, 6): 2}, {(1, 2): 0, (3, 4): 2, (5, 6): 1}, {(1, 2): 1, (3, 4): 0, (5, 6): 2}, {(1, 2): 1, (3, 4): 2, (5, 6): 0}, {(1, 2): 2, (3, 4): 0, (5, 6): 1}, {(1, 2): 2, (3, 4): 1, (5, 6): 0}] >>> K.pd_code() [[4, 2, 5, 1], [2, 6, 3, 5], [6, 4, 1, 3]] >>> K.arcs('pd') [[1, 2], [3, 4], [5, 6]]
Note that
nis not the number of different colors to be used. It can be looked upon the size of the color palette:sage: K = Knots().from_table(9, 15) sage: cols = K.colorings(13); len(cols) 156 sage: max(cols[0].values()) 12 sage: max(cols[13].values()) 9
>>> from sage.all import * >>> K = Knots().from_table(Integer(9), Integer(15)) >>> cols = K.colorings(Integer(13)); len(cols) 156 >>> max(cols[Integer(0)].values()) 12 >>> max(cols[Integer(13)].values()) 9
REFERENCES:
Chapter 3 of [Liv1993]
See also
- conway_polynomial()[source]¶
Return the Conway polynomial of
self.This is closely related to the Alexander polynomial.
See Wikipedia article Alexander_polynomial for the definition.
EXAMPLES:
sage: # needs sage.modules sage: B = BraidGroup(3) sage: L = Link(B([1, -2, 1, -2])) sage: L.conway_polynomial() -t^2 + 1 sage: Link([[1, 5, 2, 4], [3, 9, 4, 8], [5, 1, 6, 10], ....: [7, 3, 8, 2], [9, 7, 10, 6]]) Link with 1 component represented by 5 crossings sage: _.conway_polynomial() 2*t^2 + 1 sage: B = BraidGroup(4) sage: L = Link(B([1,3])) sage: L.conway_polynomial() 0
>>> from sage.all import * >>> # needs sage.modules >>> B = BraidGroup(Integer(3)) >>> L = Link(B([Integer(1), -Integer(2), Integer(1), -Integer(2)])) >>> L.conway_polynomial() -t^2 + 1 >>> Link([[Integer(1), Integer(5), Integer(2), Integer(4)], [Integer(3), Integer(9), Integer(4), Integer(8)], [Integer(5), Integer(1), Integer(6), Integer(10)], ... [Integer(7), Integer(3), Integer(8), Integer(2)], [Integer(9), Integer(7), Integer(10), Integer(6)]]) Link with 1 component represented by 5 crossings >>> _.conway_polynomial() 2*t^2 + 1 >>> B = BraidGroup(Integer(4)) >>> L = Link(B([Integer(1),Integer(3)])) >>> L.conway_polynomial() 0
See also
- determinant()[source]¶
Return the determinant of
self.EXAMPLES:
sage: # needs sage.modules sage: B = BraidGroup(4) sage: L = Link(B([-1, 2, 1, 2])) sage: L.determinant() 1 sage: B = BraidGroup(8) sage: L = Link(B([2, 4, 2, 3, 1, 2])) sage: L.determinant() 3 sage: L = Link(B([1]*16 + [2,1,2,1,2,2,2,2,2,2,2,1,2,1,2,-1,2,-2])) sage: L.determinant() 65 sage: B = BraidGroup(3) sage: Link(B([1, 2, 1, 1, 2])).determinant() 4
>>> from sage.all import * >>> # needs sage.modules >>> B = BraidGroup(Integer(4)) >>> L = Link(B([-Integer(1), Integer(2), Integer(1), Integer(2)])) >>> L.determinant() 1 >>> B = BraidGroup(Integer(8)) >>> L = Link(B([Integer(2), Integer(4), Integer(2), Integer(3), Integer(1), Integer(2)])) >>> L.determinant() 3 >>> L = Link(B([Integer(1)]*Integer(16) + [Integer(2),Integer(1),Integer(2),Integer(1),Integer(2),Integer(2),Integer(2),Integer(2),Integer(2),Integer(2),Integer(2),Integer(1),Integer(2),Integer(1),Integer(2),-Integer(1),Integer(2),-Integer(2)])) >>> L.determinant() 65 >>> B = BraidGroup(Integer(3)) >>> Link(B([Integer(1), Integer(2), Integer(1), Integer(1), Integer(2)])).determinant() 4
REFERENCES:
Definition 6.6.3 in [Cro2004]
- dowker_notation()[source]¶
Return the Dowker notation of
self.Similar to the PD code we number the components, so every crossing is represented by four numbers. We focus on the incoming entities of the under and the overcrossing. It is the pair of incoming undercrossing and the incoming overcrossing. This information at every crossing gives the Dowker notation.
OUTPUT:
A list containing the pair of incoming under cross and the incoming over cross.
EXAMPLES:
sage: L = Link([[[-1, 2, -3, 4, 5, 1, -2, 6, 7, 3, -4, -7, -6,-5]], ....: [-1, -1, -1, -1, 1, -1, 1]]) sage: L.dowker_notation() [(1, 6), (7, 2), (3, 10), (11, 4), (14, 5), (13, 8), (12, 9)] sage: B = BraidGroup(4) sage: L = Link(B([1, 2, 1, 2])) sage: L.dowker_notation() [(2, 1), (3, 5), (6, 4), (7, 9)] sage: L = Link([[1, 4, 2, 3], [4, 1, 3, 2]]) sage: L.dowker_notation() [(1, 3), (4, 2)]
>>> from sage.all import * >>> L = Link([[[-Integer(1), Integer(2), -Integer(3), Integer(4), Integer(5), Integer(1), -Integer(2), Integer(6), Integer(7), Integer(3), -Integer(4), -Integer(7), -Integer(6),-Integer(5)]], ... [-Integer(1), -Integer(1), -Integer(1), -Integer(1), Integer(1), -Integer(1), Integer(1)]]) >>> L.dowker_notation() [(1, 6), (7, 2), (3, 10), (11, 4), (14, 5), (13, 8), (12, 9)] >>> B = BraidGroup(Integer(4)) >>> L = Link(B([Integer(1), Integer(2), Integer(1), Integer(2)])) >>> L.dowker_notation() [(2, 1), (3, 5), (6, 4), (7, 9)] >>> L = Link([[Integer(1), Integer(4), Integer(2), Integer(3)], [Integer(4), Integer(1), Integer(3), Integer(2)]]) >>> L.dowker_notation() [(1, 3), (4, 2)]
- fundamental_group(presentation='wirtinger')[source]¶
Return the fundamental group of the complement of
self.INPUT:
presentation– string; one of the following:'wirtinger'– (default) the Wirtinger presentation (see Wikipedia article Link_group)'braid'– the presentation is given by the braid action on the free group (see chapter 2 of [Bir1975])
OUTPUT: a finitely presented group
EXAMPLES:
sage: L = Link([[1, 4, 3, 2], [3, 4, 1, 2]]) sage: L.fundamental_group() Finitely presented group < x0, x1, x2 | x1*x0^-1*x2^-1*x0, x2*x0*x1^-1*x0^-1 > sage: L.fundamental_group('braid') Finitely presented group < x0, x1 | 1, 1 >
>>> from sage.all import * >>> L = Link([[Integer(1), Integer(4), Integer(3), Integer(2)], [Integer(3), Integer(4), Integer(1), Integer(2)]]) >>> L.fundamental_group() Finitely presented group < x0, x1, x2 | x1*x0^-1*x2^-1*x0, x2*x0*x1^-1*x0^-1 > >>> L.fundamental_group('braid') Finitely presented group < x0, x1 | 1, 1 >
We can see, for instance, that the two presentations of the group of the figure eight knot correspond to isomorphic groups:
sage: K8 = Knot([[[1, -2, 4, -3, 2, -1, 3, -4]], [1, 1, -1, -1]]) sage: GA = K8.fundamental_group(); GA Finitely presented group < x0, x1, x2, x3 | x2*x0*x3^-1*x0^-1, x0*x2*x1^-1*x2^-1, x1*x3^-1*x2^-1*x3, x3*x1^-1*x0^-1*x1 > sage: GB = K8.fundamental_group(presentation='braid'); GB Finitely presented group < x0, x1, x2 | x1*x2^-1*x1^-1*x0*x1*x2*x1*x2^-1*x1^-1*x0^-1*x1*x2*x1^-1*x0^-1, x1*x2^-1*x1^-1*x0*x1*x2*x1^-1*x2^-1*x1^-1*x0^-1*x1*x2*x1^-1*x0*x1*x2*x1*x2^-1*x1^-1*x0^-1*x1*x2*x1^-2, x1*x2^-1*x1^-1*x0*x1*x2*x1^-1*x2^-1 > sage: GA.simplified() Finitely presented group < x0, x1 | x1^-1*x0*x1*x0^-1*x1*x0*x1^-1*x0^-1*x1*x0^-1 > sage: GB.simplified() Finitely presented group < x0, x2 | x2^-1*x0*x2^-1*x0^-1*x2*x0*x2^-1*x0*x2*x0^-1 >
>>> from sage.all import * >>> K8 = Knot([[[Integer(1), -Integer(2), Integer(4), -Integer(3), Integer(2), -Integer(1), Integer(3), -Integer(4)]], [Integer(1), Integer(1), -Integer(1), -Integer(1)]]) >>> GA = K8.fundamental_group(); GA Finitely presented group < x0, x1, x2, x3 | x2*x0*x3^-1*x0^-1, x0*x2*x1^-1*x2^-1, x1*x3^-1*x2^-1*x3, x3*x1^-1*x0^-1*x1 > >>> GB = K8.fundamental_group(presentation='braid'); GB Finitely presented group < x0, x1, x2 | x1*x2^-1*x1^-1*x0*x1*x2*x1*x2^-1*x1^-1*x0^-1*x1*x2*x1^-1*x0^-1, x1*x2^-1*x1^-1*x0*x1*x2*x1^-1*x2^-1*x1^-1*x0^-1*x1*x2*x1^-1*x0*x1*x2*x1*x2^-1*x1^-1*x0^-1*x1*x2*x1^-2, x1*x2^-1*x1^-1*x0*x1*x2*x1^-1*x2^-1 > >>> GA.simplified() Finitely presented group < x0, x1 | x1^-1*x0*x1*x0^-1*x1*x0*x1^-1*x0^-1*x1*x0^-1 > >>> GB.simplified() Finitely presented group < x0, x2 | x2^-1*x0*x2^-1*x0^-1*x2*x0*x2^-1*x0*x2*x0^-1 >
- gauss_code()[source]¶
Return the Gauss code of
self.The Gauss code is generated by the following procedure:
Number the crossings from \(1\) to \(n\).
Select a point on the knot and start moving along the component.
At each crossing, take the number of the crossing, along with sign, which is \(-\) if it is an undercrossing and \(+\) if it is a overcrossing.
EXAMPLES:
sage: L = Link([[1, 4, 2, 3], [4, 1, 3, 2]]) sage: L.gauss_code() [[-1, 2], [1, -2]] sage: B = BraidGroup(8) sage: L = Link(B([1, -2, 1, -2, -2])) sage: L.gauss_code() [[-1, 3, -4, 5], [1, -2, 4, -5, 2, -3]] sage: L = Link([[[-1, 2], [-3, 4], [1, 3, -4, -2]], [-1, -1, 1, 1]]) sage: L.gauss_code() [[-1, 2], [-3, 4], [1, 3, -4, -2]]
>>> from sage.all import * >>> L = Link([[Integer(1), Integer(4), Integer(2), Integer(3)], [Integer(4), Integer(1), Integer(3), Integer(2)]]) >>> L.gauss_code() [[-1, 2], [1, -2]] >>> B = BraidGroup(Integer(8)) >>> L = Link(B([Integer(1), -Integer(2), Integer(1), -Integer(2), -Integer(2)])) >>> L.gauss_code() [[-1, 3, -4, 5], [1, -2, 4, -5, 2, -3]] >>> L = Link([[[-Integer(1), Integer(2)], [-Integer(3), Integer(4)], [Integer(1), Integer(3), -Integer(4), -Integer(2)]], [-Integer(1), -Integer(1), Integer(1), Integer(1)]]) >>> L.gauss_code() [[-1, 2], [-3, 4], [1, 3, -4, -2]]
- genus()[source]¶
Return the genus of
self.EXAMPLES:
sage: B = BraidGroup(4) sage: L = Link(B([-1, 3, 1, 3])) sage: L.genus() 0 sage: L = Link(B([1,3])) sage: L.genus() 0 sage: B = BraidGroup(8) sage: L = Link(B([-2, 4, 1, 6, 1, 4])) sage: L.genus() 0 sage: L = Link(B([1, 2, 1, 2])) sage: L.genus() 1
>>> from sage.all import * >>> B = BraidGroup(Integer(4)) >>> L = Link(B([-Integer(1), Integer(3), Integer(1), Integer(3)])) >>> L.genus() 0 >>> L = Link(B([Integer(1),Integer(3)])) >>> L.genus() 0 >>> B = BraidGroup(Integer(8)) >>> L = Link(B([-Integer(2), Integer(4), Integer(1), Integer(6), Integer(1), Integer(4)])) >>> L.genus() 0 >>> L = Link(B([Integer(1), Integer(2), Integer(1), Integer(2)])) >>> L.genus() 1
- get_knotinfo(mirror_version=True, unique=True)[source]¶
Identify this link as an item of the KnotInfo database (if possible).
INPUT:
mirror_version– boolean (default:True); if set toFalsethe result of the method will be just the instance ofKnotInfoBase(by default the result is a tuple of the instance and an enum, see explanation of the output below)unique– boolean (default:True); this only affects the case where a unique identification is not possible. If set toFalseyou can obtain a matching list (see explanation of the output below).
OUTPUT:
If
selfis a knot, then an element of the free monoid over prime knots constructed from the KnotInfo database is returned. More explicitly this is an element ofFreeKnotInfoMonoidElement. Else a tuple(K, m)is returned whereKis an instance ofKnotInfoBaseandman instance ofSymmetryMutant(for chiral links) specifying the symmetry mutant ofKto whichselfis isotopic. The value ofmisunknownif it cannot be determined uniquely and the keyword optionunique=Falseis given.For proper links, if the orientation mutant cannot be uniquely determined, K will be a series of links gathering all links having the same unoriented name, that is an instance of
KnotInfoSeries.If
mirror_versionis set toFalsethen the result is justK(that is:mis suppressed).If it is not possible to determine a unique result a
NotImplementedErrorwill be raised. To avoid this you can setuniquetoFalse. You will get a list of matching candidates instead.Note
The identification of proper links may fail to be unique due to the following fact: In opposite to the database for knots, there are pairs of oriented mutants of an unoriented link which are isotopic to each other. For example
L5a1_0andL5a1_1is such a pair.This is because all combinatorial possible oriented mutants are listed with individual names regardless whether they are pairwise non isotopic or not. In such a case the identification is not unique and therefore a series of the links will be returned which gathers all having the same unoriented name.
To obtain the individual oriented links being isotopic to
selfuse the keywordunique(see the examples forL2a1_1andL5a1_0below).EXAMPLES:
sage: # optional - database_knotinfo sage: L = Link([[4,1,5,2], [10,4,11,3], [5,17,6,16], [7,13,8,12], ....: [18,10,19,9], [2,12,3,11], [13,21,14,20], [15,7,16,6], ....: [22,17,1,18], [8,20,9,19], [21,15,22,14]]) sage: L.get_knotinfo() KnotInfo['K11n_121m'] sage: K = KnotInfo.K10_25 sage: l = K.link() sage: l.get_knotinfo() KnotInfo['K10_25'] sage: k11 = KnotInfo.K11n_82.link() sage: k11m = k11.mirror_image() sage: k11mr = k11m.reverse() sage: k11mr.get_knotinfo() KnotInfo['K11n_82m'] sage: k11r = k11.reverse() sage: k11r.get_knotinfo() KnotInfo['K11n_82'] sage: k11rm = k11r.mirror_image() sage: k11rm.get_knotinfo() KnotInfo['K11n_82m']
>>> from sage.all import * >>> # optional - database_knotinfo >>> L = Link([[Integer(4),Integer(1),Integer(5),Integer(2)], [Integer(10),Integer(4),Integer(11),Integer(3)], [Integer(5),Integer(17),Integer(6),Integer(16)], [Integer(7),Integer(13),Integer(8),Integer(12)], ... [Integer(18),Integer(10),Integer(19),Integer(9)], [Integer(2),Integer(12),Integer(3),Integer(11)], [Integer(13),Integer(21),Integer(14),Integer(20)], [Integer(15),Integer(7),Integer(16),Integer(6)], ... [Integer(22),Integer(17),Integer(1),Integer(18)], [Integer(8),Integer(20),Integer(9),Integer(19)], [Integer(21),Integer(15),Integer(22),Integer(14)]]) >>> L.get_knotinfo() KnotInfo['K11n_121m'] >>> K = KnotInfo.K10_25 >>> l = K.link() >>> l.get_knotinfo() KnotInfo['K10_25'] >>> k11 = KnotInfo.K11n_82.link() >>> k11m = k11.mirror_image() >>> k11mr = k11m.reverse() >>> k11mr.get_knotinfo() KnotInfo['K11n_82m'] >>> k11r = k11.reverse() >>> k11r.get_knotinfo() KnotInfo['K11n_82'] >>> k11rm = k11r.mirror_image() >>> k11rm.get_knotinfo() KnotInfo['K11n_82m']
Knots with more than 13 and multi-component links having more than 11 crossings cannot be identified. In addition non prime multi-component links or even links whose HOMFLY-PT polynomial is not irreducible cannot be identified:
sage: b, = BraidGroup(2).gens() sage: Link(b**13).get_knotinfo() # optional - database_knotinfo KnotInfo['K13a_4878'] sage: Link(b**14).get_knotinfo() Traceback (most recent call last): ... NotImplementedError: this link having more than 11 crossings cannot be determined sage: Link([[1, 4, 2, 5], [3, 8, 4, 1], [5, 2, 6, 3], ....: [6, 10, 7, 9], [10, 8, 9, 7]]) Link with 2 components represented by 5 crossings sage: _.get_knotinfo() # needs sage.libs.homfly Traceback (most recent call last): ... NotImplementedError: this (possibly non prime) link cannot be determined
>>> from sage.all import * >>> b, = BraidGroup(Integer(2)).gens() >>> Link(b**Integer(13)).get_knotinfo() # optional - database_knotinfo KnotInfo['K13a_4878'] >>> Link(b**Integer(14)).get_knotinfo() Traceback (most recent call last): ... NotImplementedError: this link having more than 11 crossings cannot be determined >>> Link([[Integer(1), Integer(4), Integer(2), Integer(5)], [Integer(3), Integer(8), Integer(4), Integer(1)], [Integer(5), Integer(2), Integer(6), Integer(3)], ... [Integer(6), Integer(10), Integer(7), Integer(9)], [Integer(10), Integer(8), Integer(9), Integer(7)]]) Link with 2 components represented by 5 crossings >>> _.get_knotinfo() # needs sage.libs.homfly Traceback (most recent call last): ... NotImplementedError: this (possibly non prime) link cannot be determined
Lets identify the monster unknot:
sage: L = Link([[3,1,2,4], [8,9,1,7], [5,6,7,3], [4,18,6,5], ....: [17,19,8,18], [9,10,11,14], [10,12,13,11], ....: [12,19,15,13], [20,16,14,15], [16,20,17,2]]) sage: L.get_knotinfo() KnotInfo['K0_1']
>>> from sage.all import * >>> L = Link([[Integer(3),Integer(1),Integer(2),Integer(4)], [Integer(8),Integer(9),Integer(1),Integer(7)], [Integer(5),Integer(6),Integer(7),Integer(3)], [Integer(4),Integer(18),Integer(6),Integer(5)], ... [Integer(17),Integer(19),Integer(8),Integer(18)], [Integer(9),Integer(10),Integer(11),Integer(14)], [Integer(10),Integer(12),Integer(13),Integer(11)], ... [Integer(12),Integer(19),Integer(15),Integer(13)], [Integer(20),Integer(16),Integer(14),Integer(15)], [Integer(16),Integer(20),Integer(17),Integer(2)]]) >>> L.get_knotinfo() KnotInfo['K0_1']
Usage of option
mirror_version:sage: L.get_knotinfo(mirror_version=False) == KnotInfo.K0_1 True
>>> from sage.all import * >>> L.get_knotinfo(mirror_version=False) == KnotInfo.K0_1 True
Usage of option
unique:sage: # optional - database_knotinfo sage: l = K.link(K.items.gauss_notation) sage: l.get_knotinfo() Traceback (most recent call last): ... NotImplementedError: this link cannot be uniquely determined use keyword argument `unique` to obtain more details sage: l.get_knotinfo(unique=False) [KnotInfo['K10_25'], KnotInfo['K10_56']] sage: t = (1, -2, 1, 1, -2, 1, -2, -2) sage: l8 = Link(BraidGroup(3)(t)) sage: l8.get_knotinfo() Traceback (most recent call last): ... NotImplementedError: this link cannot be uniquely determined use keyword argument `unique` to obtain more details sage: l8.get_knotinfo(unique=False) [(<KnotInfo.L8a19_0_0: 'L8a19{0,0}'>, <SymmetryMutant.itself: 's'>), (<KnotInfo.L8a19_1_1: 'L8a19{1,1}'>, <SymmetryMutant.itself: 's'>)] sage: t = (2, -3, -3, -2, 3, 3, -2, 3, 1, -2, -2, 1) sage: l12 = Link(BraidGroup(5)(t)) sage: l12.get_knotinfo() Traceback (most recent call last): ... NotImplementedError: this link having more than 11 crossings cannot be uniquely determined use keyword argument `unique` to obtain more details sage: l12.get_knotinfo(unique=False) [(<KnotInfo.L10n36_0: 'L10n36{0}'>, <SymmetryMutant.unknown: '?'>), (<KnotInfo.L10n36_1: 'L10n36{1}'>, <SymmetryMutant.unknown: '?'>), (<KnotInfo.L10n59_0: 'L10n59{0}'>, <SymmetryMutant.itself: 's'>), (<KnotInfo.L10n59_1: 'L10n59{1}'>, <SymmetryMutant.itself: 's'>)]
>>> from sage.all import * >>> # optional - database_knotinfo >>> l = K.link(K.items.gauss_notation) >>> l.get_knotinfo() Traceback (most recent call last): ... NotImplementedError: this link cannot be uniquely determined use keyword argument `unique` to obtain more details >>> l.get_knotinfo(unique=False) [KnotInfo['K10_25'], KnotInfo['K10_56']] >>> t = (Integer(1), -Integer(2), Integer(1), Integer(1), -Integer(2), Integer(1), -Integer(2), -Integer(2)) >>> l8 = Link(BraidGroup(Integer(3))(t)) >>> l8.get_knotinfo() Traceback (most recent call last): ... NotImplementedError: this link cannot be uniquely determined use keyword argument `unique` to obtain more details >>> l8.get_knotinfo(unique=False) [(<KnotInfo.L8a19_0_0: 'L8a19{0,0}'>, <SymmetryMutant.itself: 's'>), (<KnotInfo.L8a19_1_1: 'L8a19{1,1}'>, <SymmetryMutant.itself: 's'>)] >>> t = (Integer(2), -Integer(3), -Integer(3), -Integer(2), Integer(3), Integer(3), -Integer(2), Integer(3), Integer(1), -Integer(2), -Integer(2), Integer(1)) >>> l12 = Link(BraidGroup(Integer(5))(t)) >>> l12.get_knotinfo() Traceback (most recent call last): ... NotImplementedError: this link having more than 11 crossings cannot be uniquely determined use keyword argument `unique` to obtain more details >>> l12.get_knotinfo(unique=False) [(<KnotInfo.L10n36_0: 'L10n36{0}'>, <SymmetryMutant.unknown: '?'>), (<KnotInfo.L10n36_1: 'L10n36{1}'>, <SymmetryMutant.unknown: '?'>), (<KnotInfo.L10n59_0: 'L10n59{0}'>, <SymmetryMutant.itself: 's'>), (<KnotInfo.L10n59_1: 'L10n59{1}'>, <SymmetryMutant.itself: 's'>)]
Furthermore, if the result is a complete series of oriented links having the same unoriented name (according to the note above) the option can be used to achieve more detailed information:
sage: L2a1 = Link(b**2) sage: L2a1.get_knotinfo() (Series of links L2a1, <SymmetryMutant.mixed: 'x'>) sage: L2a1.get_knotinfo(unique=False) [(<KnotInfo.L2a1_0: 'L2a1{0}'>, <SymmetryMutant.mirror_image: 'm'>), (<KnotInfo.L2a1_1: 'L2a1{1}'>, <SymmetryMutant.itself: 's'>)] sage: KnotInfo.L5a1_0.inject() Defining L5a1_0 sage: l5 = Link(L5a1_0.braid()) sage: l5.get_knotinfo() (Series of links L5a1, <SymmetryMutant.itself: 's'>) sage: _[0].inject() Defining L5a1 sage: list(L5a1) [<KnotInfo.L5a1_0: 'L5a1{0}'>, <KnotInfo.L5a1_1: 'L5a1{1}'>] sage: l5.get_knotinfo(unique=False) [(<KnotInfo.L5a1_0: 'L5a1{0}'>, <SymmetryMutant.itself: 's'>), (<KnotInfo.L5a1_1: 'L5a1{1}'>, <SymmetryMutant.itself: 's'>)]
>>> from sage.all import * >>> L2a1 = Link(b**Integer(2)) >>> L2a1.get_knotinfo() (Series of links L2a1, <SymmetryMutant.mixed: 'x'>) >>> L2a1.get_knotinfo(unique=False) [(<KnotInfo.L2a1_0: 'L2a1{0}'>, <SymmetryMutant.mirror_image: 'm'>), (<KnotInfo.L2a1_1: 'L2a1{1}'>, <SymmetryMutant.itself: 's'>)] >>> KnotInfo.L5a1_0.inject() Defining L5a1_0 >>> l5 = Link(L5a1_0.braid()) >>> l5.get_knotinfo() (Series of links L5a1, <SymmetryMutant.itself: 's'>) >>> _[Integer(0)].inject() Defining L5a1 >>> list(L5a1) [<KnotInfo.L5a1_0: 'L5a1{0}'>, <KnotInfo.L5a1_1: 'L5a1{1}'>] >>> l5.get_knotinfo(unique=False) [(<KnotInfo.L5a1_0: 'L5a1{0}'>, <SymmetryMutant.itself: 's'>), (<KnotInfo.L5a1_1: 'L5a1{1}'>, <SymmetryMutant.itself: 's'>)]
Clarifying the series around the Perko pair (Wikipedia article Perko_pair):
sage: for i in range(160, 166): # optional - database_knotinfo ....: K = Knots().from_table(10, i) ....: print('%s_%s' %(10, i), '--->', K.get_knotinfo()) 10_160 ---> KnotInfo['K10_160'] 10_161 ---> KnotInfo['K10_161m'] 10_162 ---> KnotInfo['K10_162'] 10_163 ---> KnotInfo['K10_163'] 10_164 ---> KnotInfo['K10_164'] 10_165 ---> KnotInfo['K10_165m']
>>> from sage.all import * >>> for i in range(Integer(160), Integer(166)): # optional - database_knotinfo ... K = Knots().from_table(Integer(10), i) ... print('%s_%s' %(Integer(10), i), '--->', K.get_knotinfo()) 10_160 ---> KnotInfo['K10_160'] 10_161 ---> KnotInfo['K10_161m'] 10_162 ---> KnotInfo['K10_162'] 10_163 ---> KnotInfo['K10_163'] 10_164 ---> KnotInfo['K10_164'] 10_165 ---> KnotInfo['K10_165m']
Clarifying ther Perko series against SnapPy:
sage: import snappy # optional - snappy ... sage: # optional - database_knotinfo snappy sage: from sage.knots.knotinfo import KnotInfoSeries sage: KnotInfoSeries(10, True, True) Series of knots K10 sage: _.inject() Defining K10 sage: for i in range(160, 166): ....: K = K10(i) ....: k = K.link(K.items.name, snappy=True) ....: print(k, '--->', k.sage_link().get_knotinfo()) <Link 10_160: 1 comp; 10 cross> ---> KnotInfo['K10_160'] <Link 10_161: 1 comp; 10 cross> ---> KnotInfo['K10_161m'] <Link 10_162: 1 comp; 10 cross> ---> KnotInfo['K10_161'] <Link 10_163: 1 comp; 10 cross> ---> KnotInfo['K10_162'] <Link 10_164: 1 comp; 10 cross> ---> KnotInfo['K10_163'] <Link 10_165: 1 comp; 10 cross> ---> KnotInfo['K10_164'] sage: snappy.Link('10_166') <Link 10_166: 1 comp; 10 cross> sage: _.sage_link().get_knotinfo() KnotInfo['K10_165m']
>>> from sage.all import * >>> import snappy # optional - snappy ... >>> # optional - database_knotinfo snappy >>> from sage.knots.knotinfo import KnotInfoSeries >>> KnotInfoSeries(Integer(10), True, True) Series of knots K10 >>> _.inject() Defining K10 >>> for i in range(Integer(160), Integer(166)): ... K = K10(i) ... k = K.link(K.items.name, snappy=True) ... print(k, '--->', k.sage_link().get_knotinfo()) <Link 10_160: 1 comp; 10 cross> ---> KnotInfo['K10_160'] <Link 10_161: 1 comp; 10 cross> ---> KnotInfo['K10_161m'] <Link 10_162: 1 comp; 10 cross> ---> KnotInfo['K10_161'] <Link 10_163: 1 comp; 10 cross> ---> KnotInfo['K10_162'] <Link 10_164: 1 comp; 10 cross> ---> KnotInfo['K10_163'] <Link 10_165: 1 comp; 10 cross> ---> KnotInfo['K10_164'] >>> snappy.Link('10_166') <Link 10_166: 1 comp; 10 cross> >>> _.sage_link().get_knotinfo() KnotInfo['K10_165m']
Another pair of confusion (see the corresponding Warning):
sage: # optional - database_knotinfo snappy sage: Ks10_86 = snappy.Link('10_86') sage: Ks10_83 = snappy.Link('10_83') sage: Ks10_86.sage_link().get_knotinfo(unique=False) [KnotInfo['K10_83c'], KnotInfo['K10_83m']] sage: Ks10_83.sage_link().get_knotinfo(unique=False) [KnotInfo['K10_86'], KnotInfo['K10_86r']]
>>> from sage.all import * >>> # optional - database_knotinfo snappy >>> Ks10_86 = snappy.Link('10_86') >>> Ks10_83 = snappy.Link('10_83') >>> Ks10_86.sage_link().get_knotinfo(unique=False) [KnotInfo['K10_83c'], KnotInfo['K10_83m']] >>> Ks10_83.sage_link().get_knotinfo(unique=False) [KnotInfo['K10_86'], KnotInfo['K10_86r']]
Non prime knots can be detected, as well:
sage: b = BraidGroup(4)((1, 2, 2, 2, -1, 2, 2, 2, -3, -3, -3)) sage: Kb = Knot(b) sage: Kb.get_knotinfo() KnotInfo['K3_1']^2*KnotInfo['K3_1m'] sage: K = Link([[4, 2, 5, 1], [8, 6, 9, 5], [6, 3, 7, 4], [2, 7, 3, 8], ....: [10, 15, 11, 16], [12, 21, 13, 22], [14, 11, 15, 12], [16, 9, 17, 10], ....: [18, 25, 19, 26], [20, 23, 21, 24], [22, 13, 23, 14], [24, 19, 25, 20], ....: [26, 17, 1, 18]]) sage: K.get_knotinfo() # optional - database_knotinfo, long time KnotInfo['K4_1']*KnotInfo['K9_2m']
>>> from sage.all import * >>> b = BraidGroup(Integer(4))((Integer(1), Integer(2), Integer(2), Integer(2), -Integer(1), Integer(2), Integer(2), Integer(2), -Integer(3), -Integer(3), -Integer(3))) >>> Kb = Knot(b) >>> Kb.get_knotinfo() KnotInfo['K3_1']^2*KnotInfo['K3_1m'] >>> K = Link([[Integer(4), Integer(2), Integer(5), Integer(1)], [Integer(8), Integer(6), Integer(9), Integer(5)], [Integer(6), Integer(3), Integer(7), Integer(4)], [Integer(2), Integer(7), Integer(3), Integer(8)], ... [Integer(10), Integer(15), Integer(11), Integer(16)], [Integer(12), Integer(21), Integer(13), Integer(22)], [Integer(14), Integer(11), Integer(15), Integer(12)], [Integer(16), Integer(9), Integer(17), Integer(10)], ... [Integer(18), Integer(25), Integer(19), Integer(26)], [Integer(20), Integer(23), Integer(21), Integer(24)], [Integer(22), Integer(13), Integer(23), Integer(14)], [Integer(24), Integer(19), Integer(25), Integer(20)], ... [Integer(26), Integer(17), Integer(1), Integer(18)]]) >>> K.get_knotinfo() # optional - database_knotinfo, long time KnotInfo['K4_1']*KnotInfo['K9_2m']
- homfly_polynomial(var1=None, var2=None, normalization='lm')[source]¶
Return the HOMFLY polynomial of
self.The HOMFLY polynomial \(P(K)\) of a link \(K\) is a Laurent polynomial in two variables defined using skein relations and for the unknot \(U\), we have \(P(U) = 1\).
INPUT:
var1– (default:'L') the first variable. Ifnormalizationis set toazresp.vzthe default isaresp.vvar2– (default:'M') the second variable. Ifnormalizationis set toazresp.vzthe default isznormalization– (default:lm) the system of coordinates and can be one of the following:'lm'– corresponding to the Skein relation \(L\cdot P(K _+) + L^{-1}\cdot P(K _-) + M\cdot P(K _0) = 0\)'az'– corresponding to the Skein relation \(a\cdot P(K _+) - a^{-1}\cdot P(K _-) = z \cdot P(K _0)\)'vz'– corresponding to the Skein relation \(v^{-1}\cdot P(K _+) - v\cdot P(K _-) = z \cdot P(K _0)\)
where \(P(K _+)\), \(P(K _-)\) and \(P(K _0)\) represent the HOMFLY polynomials of three links that vary only in one crossing; that is the positive, negative, or smoothed links respectively
OUTPUT: a Laurent polynomial over the integers
Note
Use the
'az'normalization to agree with the data in [KnotAtlas]Use the
'vz'normalization to agree with the data KnotInfo.EXAMPLES:
We give some examples:
sage: g = BraidGroup(2).gen(0) sage: K = Knot(g^5) sage: K.homfly_polynomial() # needs sage.libs.homfly L^-4*M^4 - 4*L^-4*M^2 + 3*L^-4 - L^-6*M^2 + 2*L^-6
>>> from sage.all import * >>> g = BraidGroup(Integer(2)).gen(Integer(0)) >>> K = Knot(g**Integer(5)) >>> K.homfly_polynomial() # needs sage.libs.homfly L^-4*M^4 - 4*L^-4*M^2 + 3*L^-4 - L^-6*M^2 + 2*L^-6
The Hopf link:
sage: L = Link([[1,4,2,3],[4,1,3,2]]) sage: L.homfly_polynomial('x', 'y') # needs sage.libs.homfly -x^-1*y + x^-1*y^-1 + x^-3*y^-1
>>> from sage.all import * >>> L = Link([[Integer(1),Integer(4),Integer(2),Integer(3)],[Integer(4),Integer(1),Integer(3),Integer(2)]]) >>> L.homfly_polynomial('x', 'y') # needs sage.libs.homfly -x^-1*y + x^-1*y^-1 + x^-3*y^-1
Another version of the Hopf link where the orientation has been changed. Therefore we substitute \(x \mapsto L^{-1}\) and \(y \mapsto M\):
sage: L = Link([[1,3,2,4], [4,2,3,1]]) sage: L.homfly_polynomial() # needs sage.libs.homfly L^3*M^-1 - L*M + L*M^-1 sage: L = Link([[1,3,2,4], [4,2,3,1]]) sage: L.homfly_polynomial(normalization='az') # needs sage.libs.homfly a^3*z^-1 - a*z - a*z^-1
>>> from sage.all import * >>> L = Link([[Integer(1),Integer(3),Integer(2),Integer(4)], [Integer(4),Integer(2),Integer(3),Integer(1)]]) >>> L.homfly_polynomial() # needs sage.libs.homfly L^3*M^-1 - L*M + L*M^-1 >>> L = Link([[Integer(1),Integer(3),Integer(2),Integer(4)], [Integer(4),Integer(2),Integer(3),Integer(1)]]) >>> L.homfly_polynomial(normalization='az') # needs sage.libs.homfly a^3*z^-1 - a*z - a*z^-1
The figure-eight knot:
sage: L = Link([[2,5,4,1], [5,3,7,6], [6,9,1,4], [9,7,3,2]]) sage: L.homfly_polynomial() # needs sage.libs.homfly -L^2 + M^2 - 1 - L^-2 sage: L.homfly_polynomial('a', 'z', 'az') # needs sage.libs.homfly a^2 - z^2 - 1 + a^-2
>>> from sage.all import * >>> L = Link([[Integer(2),Integer(5),Integer(4),Integer(1)], [Integer(5),Integer(3),Integer(7),Integer(6)], [Integer(6),Integer(9),Integer(1),Integer(4)], [Integer(9),Integer(7),Integer(3),Integer(2)]]) >>> L.homfly_polynomial() # needs sage.libs.homfly -L^2 + M^2 - 1 - L^-2 >>> L.homfly_polynomial('a', 'z', 'az') # needs sage.libs.homfly a^2 - z^2 - 1 + a^-2
The “monster” unknot:
sage: L = Link([[3,1,2,4], [8,9,1,7], [5,6,7,3], [4,18,6,5], ....: [17,19,8,18], [9,10,11,14], [10,12,13,11], ....: [12,19,15,13], [20,16,14,15], [16,20,17,2]]) sage: L.homfly_polynomial() # needs sage.libs.homfly 1
>>> from sage.all import * >>> L = Link([[Integer(3),Integer(1),Integer(2),Integer(4)], [Integer(8),Integer(9),Integer(1),Integer(7)], [Integer(5),Integer(6),Integer(7),Integer(3)], [Integer(4),Integer(18),Integer(6),Integer(5)], ... [Integer(17),Integer(19),Integer(8),Integer(18)], [Integer(9),Integer(10),Integer(11),Integer(14)], [Integer(10),Integer(12),Integer(13),Integer(11)], ... [Integer(12),Integer(19),Integer(15),Integer(13)], [Integer(20),Integer(16),Integer(14),Integer(15)], [Integer(16),Integer(20),Integer(17),Integer(2)]]) >>> L.homfly_polynomial() # needs sage.libs.homfly 1
Comparison with KnotInfo:
sage: # needs sage.libs.homfly sage: KI = K.get_knotinfo(mirror_version=False); KI <KnotInfo.K5_1: '5_1'> sage: K.homfly_polynomial(normalization='vz') == KI.homfly_polynomial() True
>>> from sage.all import * >>> # needs sage.libs.homfly >>> KI = K.get_knotinfo(mirror_version=False); KI <KnotInfo.K5_1: '5_1'> >>> K.homfly_polynomial(normalization='vz') == KI.homfly_polynomial() True
The knot \(9_6\):
sage: # needs sage.libs.homfly sage: B = BraidGroup(3) sage: K = Knot(B([-1,-1,-1,-1,-1,-1,-2,1,-2,-2])) sage: K.homfly_polynomial() L^10*M^4 - L^8*M^6 - 3*L^10*M^2 + 4*L^8*M^4 + L^6*M^6 + L^10 - 3*L^8*M^2 - 5*L^6*M^4 - L^8 + 7*L^6*M^2 - 3*L^6 sage: K.homfly_polynomial('a', 'z', normalization='az') -a^10*z^4 + a^8*z^6 - 3*a^10*z^2 + 4*a^8*z^4 + a^6*z^6 - a^10 + 3*a^8*z^2 + 5*a^6*z^4 - a^8 + 7*a^6*z^2 + 3*a^6
>>> from sage.all import * >>> # needs sage.libs.homfly >>> B = BraidGroup(Integer(3)) >>> K = Knot(B([-Integer(1),-Integer(1),-Integer(1),-Integer(1),-Integer(1),-Integer(1),-Integer(2),Integer(1),-Integer(2),-Integer(2)])) >>> K.homfly_polynomial() L^10*M^4 - L^8*M^6 - 3*L^10*M^2 + 4*L^8*M^4 + L^6*M^6 + L^10 - 3*L^8*M^2 - 5*L^6*M^4 - L^8 + 7*L^6*M^2 - 3*L^6 >>> K.homfly_polynomial('a', 'z', normalization='az') -a^10*z^4 + a^8*z^6 - 3*a^10*z^2 + 4*a^8*z^4 + a^6*z^6 - a^10 + 3*a^8*z^2 + 5*a^6*z^4 - a^8 + 7*a^6*z^2 + 3*a^6
REFERENCES:
- is_alternating()[source]¶
Return whether the given knot diagram is alternating.
Alternating diagram implies every overcross is followed by an undercross or the vice-versa.
We look at the Gauss code if the sign is alternating,
Trueis returned else the knot is not alternatingFalseis returned.Warning
This does not check if a knot admits an alternating diagram or not. Thus, this term is used differently than in some of the literature, such as in Hoste-Thistlethwaite table.
Note
Links with more than one component are considered to not be alternating (knots) even when such a diagram exists.
EXAMPLES:
sage: B = BraidGroup(4) sage: L = Link(B([-1, -1, -1, -1])) sage: L.is_alternating() False sage: L = Link(B([1, -2, -1, 2])) sage: L.is_alternating() False sage: L = Link(B([-1, 3, 1, 3, 2])) sage: L.is_alternating() False sage: L = Link(B([1]*16 + [2,1,2,1,2,2,2,2,2,2,2,1,2,1,2,-1,2,-2])) sage: L.is_alternating() False sage: L = Link(B([-1,2,-1,2])) sage: L.is_alternating() True
>>> from sage.all import * >>> B = BraidGroup(Integer(4)) >>> L = Link(B([-Integer(1), -Integer(1), -Integer(1), -Integer(1)])) >>> L.is_alternating() False >>> L = Link(B([Integer(1), -Integer(2), -Integer(1), Integer(2)])) >>> L.is_alternating() False >>> L = Link(B([-Integer(1), Integer(3), Integer(1), Integer(3), Integer(2)])) >>> L.is_alternating() False >>> L = Link(B([Integer(1)]*Integer(16) + [Integer(2),Integer(1),Integer(2),Integer(1),Integer(2),Integer(2),Integer(2),Integer(2),Integer(2),Integer(2),Integer(2),Integer(1),Integer(2),Integer(1),Integer(2),-Integer(1),Integer(2),-Integer(2)])) >>> L.is_alternating() False >>> L = Link(B([-Integer(1),Integer(2),-Integer(1),Integer(2)])) >>> L.is_alternating() True
We give the \(5_2\) knot with an alternating diagram and a non-alternating diagram:
sage: K5_2 = Link([[1, 4, 2, 5], [3, 8, 4, 9], [5, 10, 6, 1], ....: [7, 2, 8, 3], [9, 6, 10, 7]]) sage: K5_2.is_alternating() True sage: K5_2b = Link(K5_2.braid()) sage: K5_2b.is_alternating() False
>>> from sage.all import * >>> K5_2 = Link([[Integer(1), Integer(4), Integer(2), Integer(5)], [Integer(3), Integer(8), Integer(4), Integer(9)], [Integer(5), Integer(10), Integer(6), Integer(1)], ... [Integer(7), Integer(2), Integer(8), Integer(3)], [Integer(9), Integer(6), Integer(10), Integer(7)]]) >>> K5_2.is_alternating() True >>> K5_2b = Link(K5_2.braid()) >>> K5_2b.is_alternating() False
- is_colorable(n=None)[source]¶
Return whether the link is
n-colorable.A link is
n-colorable if its arcs can be painted withncolours, labeled from0ton - 1, in such a way that at any crossing, the average of the indices of the undercrossings equals twice the index of the overcrossing.INPUT:
n– the number of colors to consider (if ommitted the value of the determinant ofselfwill be taken)
EXAMPLES:
We show that the trefoil knot is 3-colorable:
sage: K = Link([[[1, -2, 3, -1, 2, -3]], [1, 1, 1]]) sage: K.is_colorable(3) # needs sage.libs.pari sage.modules True
>>> from sage.all import * >>> K = Link([[[Integer(1), -Integer(2), Integer(3), -Integer(1), Integer(2), -Integer(3)]], [Integer(1), Integer(1), Integer(1)]]) >>> K.is_colorable(Integer(3)) # needs sage.libs.pari sage.modules True
But the figure eight knot is not:
sage: K8 = Link([[[1, -2, 4, -3, 2, -1, 3, -4]], [1, 1, -1, -1]]) sage: K8.is_colorable(3) # needs sage.libs.pari sage.modules False
>>> from sage.all import * >>> K8 = Link([[[Integer(1), -Integer(2), Integer(4), -Integer(3), Integer(2), -Integer(1), Integer(3), -Integer(4)]], [Integer(1), Integer(1), -Integer(1), -Integer(1)]]) >>> K8.is_colorable(Integer(3)) # needs sage.libs.pari sage.modules False
But it is colorable with respect to the value of its determinant:
sage: K8.determinant() 5 sage: K8.is_colorable() True
>>> from sage.all import * >>> K8.determinant() 5 >>> K8.is_colorable() True
An examples with non prime determinant:
sage: K = Knots().from_table(6, 1) sage: K.determinant() 9 sage: K.is_colorable() True
>>> from sage.all import * >>> K = Knots().from_table(Integer(6), Integer(1)) >>> K.determinant() 9 >>> K.is_colorable() True
REFERENCES:
Chapter 3 of [Liv1993]
See also
- is_isotopic(other)[source]¶
Check whether
selfis isotopic toother.INPUT:
other– another instance ofLink
EXAMPLES:
sage: l1 = Link([[2, 9, 3, 10], [4, 13, 5, 14], [6, 11, 7, 12], ....: [8, 1, 9, 2], [10, 7, 11, 8], [12, 5, 13, 6], ....: [14, 3, 1, 4]]) sage: l2 = Link([[1, 8, 2, 9], [9, 2, 10, 3], [3, 14, 4, 1], ....: [13, 4, 14, 5], [5, 12, 6, 13], [11, 6, 12, 7], ....: [7, 10, 8, 11]]) sage: l1.is_isotopic(l2) True sage: l3 = l2.mirror_image() sage: l1.is_isotopic(l3) False sage: # optional - database_knotinfo sage: L = KnotInfo.L7a7_0_0 sage: L.series(oriented=True).inject() Defining L7a7 sage: L == L7a7(0) True sage: l = L.link() sage: l.is_isotopic(L7a7(1).link()) Traceback (most recent call last): ... NotImplementedError: comparison not possible! sage: l.is_isotopic(L7a7(2).link()) True sage: l.is_isotopic(L7a7(3).link()) False
>>> from sage.all import * >>> l1 = Link([[Integer(2), Integer(9), Integer(3), Integer(10)], [Integer(4), Integer(13), Integer(5), Integer(14)], [Integer(6), Integer(11), Integer(7), Integer(12)], ... [Integer(8), Integer(1), Integer(9), Integer(2)], [Integer(10), Integer(7), Integer(11), Integer(8)], [Integer(12), Integer(5), Integer(13), Integer(6)], ... [Integer(14), Integer(3), Integer(1), Integer(4)]]) >>> l2 = Link([[Integer(1), Integer(8), Integer(2), Integer(9)], [Integer(9), Integer(2), Integer(10), Integer(3)], [Integer(3), Integer(14), Integer(4), Integer(1)], ... [Integer(13), Integer(4), Integer(14), Integer(5)], [Integer(5), Integer(12), Integer(6), Integer(13)], [Integer(11), Integer(6), Integer(12), Integer(7)], ... [Integer(7), Integer(10), Integer(8), Integer(11)]]) >>> l1.is_isotopic(l2) True >>> l3 = l2.mirror_image() >>> l1.is_isotopic(l3) False >>> # optional - database_knotinfo >>> L = KnotInfo.L7a7_0_0 >>> L.series(oriented=True).inject() Defining L7a7 >>> L == L7a7(Integer(0)) True >>> l = L.link() >>> l.is_isotopic(L7a7(Integer(1)).link()) Traceback (most recent call last): ... NotImplementedError: comparison not possible! >>> l.is_isotopic(L7a7(Integer(2)).link()) True >>> l.is_isotopic(L7a7(Integer(3)).link()) False
Using verbosity:
sage: set_verbose(1) sage: l1.is_isotopic(l2) verbose 1 (... link.py, is_isotopic) identified by KnotInfo (KnotInfo.K7_2, SymmetryMutant.mirror_image) True sage: l1.is_isotopic(l3) verbose 1 (... link.py, is_isotopic) different Homfly-PT polynomials False sage: set_verbose(0)
>>> from sage.all import * >>> set_verbose(Integer(1)) >>> l1.is_isotopic(l2) verbose 1 (... link.py, is_isotopic) identified by KnotInfo (KnotInfo.K7_2, SymmetryMutant.mirror_image) True >>> l1.is_isotopic(l3) verbose 1 (... link.py, is_isotopic) different Homfly-PT polynomials False >>> set_verbose(Integer(0))
- is_knot()[source]¶
Return
Trueifselfis a knot.Every knot is a link but the converse is not true.
EXAMPLES:
sage: B = BraidGroup(4) sage: L = Link(B([1, 3, 1, -3])) sage: L.is_knot() False sage: B = BraidGroup(8) sage: L = Link(B([1, 2, 3, 4, 5, 6])) sage: L.is_knot() True
>>> from sage.all import * >>> B = BraidGroup(Integer(4)) >>> L = Link(B([Integer(1), Integer(3), Integer(1), -Integer(3)])) >>> L.is_knot() False >>> B = BraidGroup(Integer(8)) >>> L = Link(B([Integer(1), Integer(2), Integer(3), Integer(4), Integer(5), Integer(6)])) >>> L.is_knot() True
- jones_polynomial(variab=None, skein_normalization=False, algorithm='jonesrep')[source]¶
Return the Jones polynomial of
self.The normalization is so that the unknot has Jones polynomial \(1\). If
skein_normalizationisTrue, the variable of the result is replaced by a itself to the power of \(4\), so that the result agrees with the conventions of [Lic1997] (which in particular differs slightly from the conventions used otherwise in this class), had one used the conventional Kauffman bracket variable notation directly.If
variabisNonereturn a polynomial in the variable \(A\) or \(t\), depending on the valueskein_normalization. In particular, ifskein_normalizationisFalse, return the result in terms of the variable \(t\), also used in [Lic1997].ALGORITHM:
The calculation goes through one of two possible algorithms, depending on the value of
algorithm. Possible values are'jonesrep'which uses the Jones representation of a braid representation ofselfto compute the polynomial of the trace closure of the braid, andstatesumwhich recursively computes the Kauffman bracket ofself. Depending on how the link is given, there might be significant time gains in using one over the other. When the trace closure of the braid isself, the algorithms give the same result.INPUT:
variab– variable (default:None); the variable in the resulting polynomial; if unspecified, use either a default variable in \(\ZZ[A,A^{-1}]\) or the variable \(t\) in the symbolic ringskein_normalization– boolean (default:False); determines the variable of the resulting polynomialalgorithm– string (default:'jonesrep'); algorithm to use and can be one of the following:'jonesrep'– use the Jones representation of the braid representation'statesum'– recursively computes the Kauffman bracket
OUTPUT:
If
skein_normalizationifFalse, this returns an element in the symbolic ring as the Jones polynomial of the link might have fractional powers when the link is not a knot. Otherwise the result is a Laurent polynomial invariab.EXAMPLES:
The unknot:
sage: B = BraidGroup(9) sage: b = B([1, 2, 3, 4, 5, 6, 7, 8]) sage: Link(b).jones_polynomial() 1
>>> from sage.all import * >>> B = BraidGroup(Integer(9)) >>> b = B([Integer(1), Integer(2), Integer(3), Integer(4), Integer(5), Integer(6), Integer(7), Integer(8)]) >>> Link(b).jones_polynomial() 1
The “monster” unknot:
sage: L = Link([[3,1,2,4],[8,9,1,7],[5,6,7,3],[4,18,6,5], ....: [17,19,8,18],[9,10,11,14],[10,12,13,11], ....: [12,19,15,13],[20,16,14,15],[16,20,17,2]]) sage: L.jones_polynomial() # needs sage.symbolic 1
>>> from sage.all import * >>> L = Link([[Integer(3),Integer(1),Integer(2),Integer(4)],[Integer(8),Integer(9),Integer(1),Integer(7)],[Integer(5),Integer(6),Integer(7),Integer(3)],[Integer(4),Integer(18),Integer(6),Integer(5)], ... [Integer(17),Integer(19),Integer(8),Integer(18)],[Integer(9),Integer(10),Integer(11),Integer(14)],[Integer(10),Integer(12),Integer(13),Integer(11)], ... [Integer(12),Integer(19),Integer(15),Integer(13)],[Integer(20),Integer(16),Integer(14),Integer(15)],[Integer(16),Integer(20),Integer(17),Integer(2)]]) >>> L.jones_polynomial() # needs sage.symbolic 1
The Ochiai unknot:
sage: L = Link([[[1,-2,-3,-8,-12,13,-14,15,-7,-1,2,-4,10,11,-13,12, ....: -11,-16,4,3,-5,6,-9,7,-15,14,16,-10,8,9,-6,5]], ....: [-1,-1,1,1,1,1,-1,1,1,-1,1,-1,-1,-1,-1,-1]]) sage: L.jones_polynomial() # long time # needs sage.symbolic 1
>>> from sage.all import * >>> L = Link([[[Integer(1),-Integer(2),-Integer(3),-Integer(8),-Integer(12),Integer(13),-Integer(14),Integer(15),-Integer(7),-Integer(1),Integer(2),-Integer(4),Integer(10),Integer(11),-Integer(13),Integer(12), ... -Integer(11),-Integer(16),Integer(4),Integer(3),-Integer(5),Integer(6),-Integer(9),Integer(7),-Integer(15),Integer(14),Integer(16),-Integer(10),Integer(8),Integer(9),-Integer(6),Integer(5)]], ... [-Integer(1),-Integer(1),Integer(1),Integer(1),Integer(1),Integer(1),-Integer(1),Integer(1),Integer(1),-Integer(1),Integer(1),-Integer(1),-Integer(1),-Integer(1),-Integer(1),-Integer(1)]]) >>> L.jones_polynomial() # long time # needs sage.symbolic 1
Two unlinked unknots:
sage: B = BraidGroup(4) sage: b = B([1, 3]) sage: Link(b).jones_polynomial() # needs sage.symbolic -sqrt(t) - 1/sqrt(t)
>>> from sage.all import * >>> B = BraidGroup(Integer(4)) >>> b = B([Integer(1), Integer(3)]) >>> Link(b).jones_polynomial() # needs sage.symbolic -sqrt(t) - 1/sqrt(t)
The Hopf link:
sage: B = BraidGroup(2) sage: b = B([-1,-1]) sage: Link(b).jones_polynomial() # needs sage.symbolic -1/sqrt(t) - 1/t^(5/2)
>>> from sage.all import * >>> B = BraidGroup(Integer(2)) >>> b = B([-Integer(1),-Integer(1)]) >>> Link(b).jones_polynomial() # needs sage.symbolic -1/sqrt(t) - 1/t^(5/2)
Different representations of the trefoil and one of its mirror:
sage: B = BraidGroup(2) sage: b = B([-1, -1, -1]) sage: Link(b).jones_polynomial(skein_normalization=True) -A^-16 + A^-12 + A^-4 sage: Link(b).jones_polynomial() 1/t + 1/t^3 - 1/t^4 sage: B = BraidGroup(3) sage: b = B([-1, -2, -1, -2]) sage: Link(b).jones_polynomial(skein_normalization=True) -A^-16 + A^-12 + A^-4 sage: R.<x> = LaurentPolynomialRing(GF(2)) sage: Link(b).jones_polynomial(skein_normalization=True, variab=x) x^-16 + x^-12 + x^-4 sage: B = BraidGroup(3) sage: b = B([1, 2, 1, 2]) sage: Link(b).jones_polynomial(skein_normalization=True) A^4 + A^12 - A^16
>>> from sage.all import * >>> B = BraidGroup(Integer(2)) >>> b = B([-Integer(1), -Integer(1), -Integer(1)]) >>> Link(b).jones_polynomial(skein_normalization=True) -A^-16 + A^-12 + A^-4 >>> Link(b).jones_polynomial() 1/t + 1/t^3 - 1/t^4 >>> B = BraidGroup(Integer(3)) >>> b = B([-Integer(1), -Integer(2), -Integer(1), -Integer(2)]) >>> Link(b).jones_polynomial(skein_normalization=True) -A^-16 + A^-12 + A^-4 >>> R = LaurentPolynomialRing(GF(Integer(2)), names=('x',)); (x,) = R._first_ngens(1) >>> Link(b).jones_polynomial(skein_normalization=True, variab=x) x^-16 + x^-12 + x^-4 >>> B = BraidGroup(Integer(3)) >>> b = B([Integer(1), Integer(2), Integer(1), Integer(2)]) >>> Link(b).jones_polynomial(skein_normalization=True) A^4 + A^12 - A^16
\(K11n42\) (the mirror of the “Kinoshita-Terasaka” knot) and \(K11n34\) (the mirror of the “Conway” knot) in [KnotAtlas]:
sage: B = BraidGroup(4) sage: K11n42 = Link(B([1, -2, 3, -2, 3, -2, -2, -1, 2, -3, -3, 2, 2])) sage: K11n34 = Link(B([1, 1, 2, -3, 2, -3, 1, -2, -2, -3, -3])) sage: bool(K11n42.jones_polynomial() == K11n34.jones_polynomial()) # needs sage.symbolic True
>>> from sage.all import * >>> B = BraidGroup(Integer(4)) >>> K11n42 = Link(B([Integer(1), -Integer(2), Integer(3), -Integer(2), Integer(3), -Integer(2), -Integer(2), -Integer(1), Integer(2), -Integer(3), -Integer(3), Integer(2), Integer(2)])) >>> K11n34 = Link(B([Integer(1), Integer(1), Integer(2), -Integer(3), Integer(2), -Integer(3), Integer(1), -Integer(2), -Integer(2), -Integer(3), -Integer(3)])) >>> bool(K11n42.jones_polynomial() == K11n34.jones_polynomial()) # needs sage.symbolic True
The two algorithms for computation give the same result when the trace closure of the braid representation is the link itself:
sage: # needs sage.symbolic sage: L = Link([[[-1, 2, -3, 4, 5, 1, -2, 6, 7, 3, -4, -7, -6, -5]], ....: [-1, -1, -1, -1, 1, -1, 1]]) sage: jonesrep = L.jones_polynomial(algorithm='jonesrep') sage: statesum = L.jones_polynomial(algorithm='statesum') sage: bool(jonesrep == statesum) True
>>> from sage.all import * >>> # needs sage.symbolic >>> L = Link([[[-Integer(1), Integer(2), -Integer(3), Integer(4), Integer(5), Integer(1), -Integer(2), Integer(6), Integer(7), Integer(3), -Integer(4), -Integer(7), -Integer(6), -Integer(5)]], ... [-Integer(1), -Integer(1), -Integer(1), -Integer(1), Integer(1), -Integer(1), Integer(1)]]) >>> jonesrep = L.jones_polynomial(algorithm='jonesrep') >>> statesum = L.jones_polynomial(algorithm='statesum') >>> bool(jonesrep == statesum) True
When we have thrown away unknots so that the trace closure of the braid is not necessarily the link itself, this is only true up to a power of the Jones polynomial of the unknot:
sage: B = BraidGroup(3) sage: b = B([1]) sage: L = Link(b) sage: b.components_in_closure() 2 sage: L.number_of_components() 1 sage: b.jones_polynomial() # needs sage.symbolic -sqrt(t) - 1/sqrt(t) sage: L.jones_polynomial() # needs sage.symbolic 1 sage: L.jones_polynomial(algorithm='statesum') # needs sage.symbolic 1
>>> from sage.all import * >>> B = BraidGroup(Integer(3)) >>> b = B([Integer(1)]) >>> L = Link(b) >>> b.components_in_closure() 2 >>> L.number_of_components() 1 >>> b.jones_polynomial() # needs sage.symbolic -sqrt(t) - 1/sqrt(t) >>> L.jones_polynomial() # needs sage.symbolic 1 >>> L.jones_polynomial(algorithm='statesum') # needs sage.symbolic 1
- khovanov_homology(ring=Integer Ring, height=None, degree=None)[source]¶
Return the Khovanov homology of the link.
INPUT:
ring– (default:ZZ) the coefficient ringheight– the height of the homology to compute, if not specified, all the heights are computeddegree– the degree of the homology to compute, if not specified, all the degrees are computed
OUTPUT:
The Khovanov homology of the Link. It is given as a dictionary whose keys are the different heights. For each height, the homology is given as another dictionary whose keys are the degrees.
EXAMPLES:
sage: K = Link([[[1, -2, 3, -1, 2, -3]],[-1, -1, -1]]) sage: K.khovanov_homology() # needs sage.modules {-9: {-3: Z}, -7: {-3: 0, -2: C2}, -5: {-3: 0, -2: Z, -1: 0, 0: 0}, -3: {-3: 0, -2: 0, -1: 0, 0: Z}, -1: {0: Z}}
>>> from sage.all import * >>> K = Link([[[Integer(1), -Integer(2), Integer(3), -Integer(1), Integer(2), -Integer(3)]],[-Integer(1), -Integer(1), -Integer(1)]]) >>> K.khovanov_homology() # needs sage.modules {-9: {-3: Z}, -7: {-3: 0, -2: C2}, -5: {-3: 0, -2: Z, -1: 0, 0: 0}, -3: {-3: 0, -2: 0, -1: 0, 0: Z}, -1: {0: Z}}
The figure eight knot:
sage: L = Link([[1, 6, 2, 7], [5, 2, 6, 3], [3, 1, 4, 8], [7, 5, 8, 4]]) sage: L.khovanov_homology(height=-1) # needs sage.modules {-1: {-2: 0, -1: Z, 0: Z, 1: 0, 2: 0}}
>>> from sage.all import * >>> L = Link([[Integer(1), Integer(6), Integer(2), Integer(7)], [Integer(5), Integer(2), Integer(6), Integer(3)], [Integer(3), Integer(1), Integer(4), Integer(8)], [Integer(7), Integer(5), Integer(8), Integer(4)]]) >>> L.khovanov_homology(height=-Integer(1)) # needs sage.modules {-1: {-2: 0, -1: Z, 0: Z, 1: 0, 2: 0}}
The Hopf link:
sage: # needs sage.modules sage: B = BraidGroup(2) sage: b = B([1, 1]) sage: K = Link(b) sage: K.khovanov_homology(degree=2) {2: {2: 0}, 4: {2: Z}, 6: {2: Z}}
>>> from sage.all import * >>> # needs sage.modules >>> B = BraidGroup(Integer(2)) >>> b = B([Integer(1), Integer(1)]) >>> K = Link(b) >>> K.khovanov_homology(degree=Integer(2)) {2: {2: 0}, 4: {2: Z}, 6: {2: Z}}
- khovanov_polynomial(var1='q', var2='t', base_ring=Integer Ring)[source]¶
Return the Khovanov polynomial of
self.This is the Poincaré polynomial of the Khovanov homology.
INPUT:
var1– (default:'q') the first variable. Its exponents give the (torsion free) rank of the height of Khovanov homologyvar2– (default:'t') the second variable. Its exponents give the (torsion free) rank of the degree of Khovanov homologybase_ring– (default:ZZ) the ring of the polynomial’s coefficients
OUTPUT:
A two variate Laurent Polynomial over the
base_ring, more precisely an instance ofLaurentPolynomial.EXAMPLES:
sage: K = Link([[[1, -2, 3, -1, 2, -3]],[-1, -1, -1]]) sage: K.khovanov_polynomial() # needs sage.modules q^-1 + q^-3 + q^-5*t^-2 + q^-9*t^-3 sage: K.khovanov_polynomial(base_ring=GF(2)) # needs sage.modules q^-1 + q^-3 + q^-5*t^-2 + q^-7*t^-2 + q^-9*t^-3
>>> from sage.all import * >>> K = Link([[[Integer(1), -Integer(2), Integer(3), -Integer(1), Integer(2), -Integer(3)]],[-Integer(1), -Integer(1), -Integer(1)]]) >>> K.khovanov_polynomial() # needs sage.modules q^-1 + q^-3 + q^-5*t^-2 + q^-9*t^-3 >>> K.khovanov_polynomial(base_ring=GF(Integer(2))) # needs sage.modules q^-1 + q^-3 + q^-5*t^-2 + q^-7*t^-2 + q^-9*t^-3
The figure eight knot:
sage: L = Link([[1, 6, 2, 7], [5, 2, 6, 3], [3, 1, 4, 8], [7, 5, 8, 4]]) sage: L.khovanov_polynomial(var1='p') # needs sage.modules p^5*t^2 + p*t + p + p^-1 + p^-1*t^-1 + p^-5*t^-2 sage: L.khovanov_polynomial(var1='p', var2='s', base_ring=GF(4)) # needs sage.modules sage.rings.finite_rings p^5*s^2 + p^3*s^2 + p*s + p + p^-1 + p^-1*s^-1 + p^-3*s^-1 + p^-5*s^-2
>>> from sage.all import * >>> L = Link([[Integer(1), Integer(6), Integer(2), Integer(7)], [Integer(5), Integer(2), Integer(6), Integer(3)], [Integer(3), Integer(1), Integer(4), Integer(8)], [Integer(7), Integer(5), Integer(8), Integer(4)]]) >>> L.khovanov_polynomial(var1='p') # needs sage.modules p^5*t^2 + p*t + p + p^-1 + p^-1*t^-1 + p^-5*t^-2 >>> L.khovanov_polynomial(var1='p', var2='s', base_ring=GF(Integer(4))) # needs sage.modules sage.rings.finite_rings p^5*s^2 + p^3*s^2 + p*s + p + p^-1 + p^-1*s^-1 + p^-3*s^-1 + p^-5*s^-2
The Hopf link:
sage: B = BraidGroup(2) sage: b = B([1, 1]) sage: K = Link(b) sage: K.khovanov_polynomial() # needs sage.modules q^6*t^2 + q^4*t^2 + q^2 + 1
>>> from sage.all import * >>> B = BraidGroup(Integer(2)) >>> b = B([Integer(1), Integer(1)]) >>> K = Link(b) >>> K.khovanov_polynomial() # needs sage.modules q^6*t^2 + q^4*t^2 + q^2 + 1
See also
- links_gould_polynomial(varnames='t0, t1')[source]¶
Return the Links-Gould polynomial of
self. See [MW2012], section 3 and references given there. See also the docstring oflinks_gould_polynomial().INPUT:
varnames– string (default:'t0, t1')
OUTPUT: a Laurent polynomial in the given variable names
EXAMPLES:
sage: Hopf = Link([[1, 3, 2, 4], [4, 2, 3, 1]]) sage: Hopf.links_gould_polynomial() -1 + t1^-1 + t0^-1 - t0^-1*t1^-1
>>> from sage.all import * >>> Hopf = Link([[Integer(1), Integer(3), Integer(2), Integer(4)], [Integer(4), Integer(2), Integer(3), Integer(1)]]) >>> Hopf.links_gould_polynomial() -1 + t1^-1 + t0^-1 - t0^-1*t1^-1
- mirror_image()[source]¶
Return the mirror image of
self.EXAMPLES:
sage: g = BraidGroup(2).gen(0) sage: K = Link(g^3) sage: K2 = K.mirror_image(); K2 Link with 1 component represented by 3 crossings sage: K2.braid() s^-3
>>> from sage.all import * >>> g = BraidGroup(Integer(2)).gen(Integer(0)) >>> K = Link(g**Integer(3)) >>> K2 = K.mirror_image(); K2 Link with 1 component represented by 3 crossings >>> K2.braid() s^-3
sage: K = Knot([[[1, -2, 3, -1, 2, -3]], [1, 1, 1]]) sage: K2 = K.mirror_image(); K2 Knot represented by 3 crossings sage: K.pd_code() [[4, 2, 5, 1], [2, 6, 3, 5], [6, 4, 1, 3]] sage: K2.pd_code() [[4, 1, 5, 2], [2, 5, 3, 6], [6, 3, 1, 4]]
>>> from sage.all import * >>> K = Knot([[[Integer(1), -Integer(2), Integer(3), -Integer(1), Integer(2), -Integer(3)]], [Integer(1), Integer(1), Integer(1)]]) >>> K2 = K.mirror_image(); K2 Knot represented by 3 crossings >>> K.pd_code() [[4, 2, 5, 1], [2, 6, 3, 5], [6, 4, 1, 3]] >>> K2.pd_code() [[4, 1, 5, 2], [2, 5, 3, 6], [6, 3, 1, 4]]
- number_of_components()[source]¶
Return the number of connected components of
self.OUTPUT: number of connected components
EXAMPLES:
sage: B = BraidGroup(4) sage: L = Link(B([-1, 3, 1, 3])) sage: L.number_of_components() 4 sage: B = BraidGroup(8) sage: L = Link(B([-2, 4, 1, 6, 1, 4])) sage: L.number_of_components() 5 sage: L = Link(B([1, 2, 1, 2])) sage: L.number_of_components() 1 sage: L = Link(B.one()) sage: L.number_of_components() 1
>>> from sage.all import * >>> B = BraidGroup(Integer(4)) >>> L = Link(B([-Integer(1), Integer(3), Integer(1), Integer(3)])) >>> L.number_of_components() 4 >>> B = BraidGroup(Integer(8)) >>> L = Link(B([-Integer(2), Integer(4), Integer(1), Integer(6), Integer(1), Integer(4)])) >>> L.number_of_components() 5 >>> L = Link(B([Integer(1), Integer(2), Integer(1), Integer(2)])) >>> L.number_of_components() 1 >>> L = Link(B.one()) >>> L.number_of_components() 1
- omega_signature(omega)[source]¶
Compute the \(\omega\)-signature of
self.INPUT:
\(\omega\) – a complex number of modulus 1; this is assumed to be coercible to
QQbar
This is defined as the signature of the Hermitian matrix
\[(1 - \omega) V + (1 - \omega^{-1}) V^{t},\]where \(V\) is the
Seifert matrix, as explained on page 122 of [Liv1993].According to [Con2018], this is also known as the Levine-Tristram signature, the equivariant signature or the Tristram-Levine signature.
See also
EXAMPLES:
sage: # needs sage.modules sage.rings.number_field sage: B = BraidGroup(4) sage: K = Knot(B([1,1,1,2,-1,2,-3,2,-3])) sage: omega = QQbar.zeta(3) sage: K.omega_signature(omega) -2
>>> from sage.all import * >>> # needs sage.modules sage.rings.number_field >>> B = BraidGroup(Integer(4)) >>> K = Knot(B([Integer(1),Integer(1),Integer(1),Integer(2),-Integer(1),Integer(2),-Integer(3),Integer(2),-Integer(3)])) >>> omega = QQbar.zeta(Integer(3)) >>> K.omega_signature(omega) -2
- orientation()[source]¶
Return the orientation of the crossings of the link diagram of
self.EXAMPLES:
sage: L = Link([[1, 2, 5, 4], [3, 7, 6, 5], [4, 6, 9, 8], [7, 11, 10, 9], ....: [8, 10, 13, 1], [11, 3, 2, 13]]) sage: L.orientation() [-1, 1, -1, 1, -1, 1] sage: L = Link([[1, 6, 2, 7], [7, 2, 8, 3], [3, 10, 4, 11], [11, 4, 12, 5], ....: [14, 6, 1, 5], [13, 8, 14, 9], [12, 10, 13, 9]]) sage: L.orientation() [-1, -1, -1, -1, 1, -1, 1] sage: L = Link([[1, 3, 3, 2], [2, 5, 5, 4], [4, 7, 7, 1]]) sage: L.orientation() [-1, -1, -1]
>>> from sage.all import * >>> L = Link([[Integer(1), Integer(2), Integer(5), Integer(4)], [Integer(3), Integer(7), Integer(6), Integer(5)], [Integer(4), Integer(6), Integer(9), Integer(8)], [Integer(7), Integer(11), Integer(10), Integer(9)], ... [Integer(8), Integer(10), Integer(13), Integer(1)], [Integer(11), Integer(3), Integer(2), Integer(13)]]) >>> L.orientation() [-1, 1, -1, 1, -1, 1] >>> L = Link([[Integer(1), Integer(6), Integer(2), Integer(7)], [Integer(7), Integer(2), Integer(8), Integer(3)], [Integer(3), Integer(10), Integer(4), Integer(11)], [Integer(11), Integer(4), Integer(12), Integer(5)], ... [Integer(14), Integer(6), Integer(1), Integer(5)], [Integer(13), Integer(8), Integer(14), Integer(9)], [Integer(12), Integer(10), Integer(13), Integer(9)]]) >>> L.orientation() [-1, -1, -1, -1, 1, -1, 1] >>> L = Link([[Integer(1), Integer(3), Integer(3), Integer(2)], [Integer(2), Integer(5), Integer(5), Integer(4)], [Integer(4), Integer(7), Integer(7), Integer(1)]]) >>> L.orientation() [-1, -1, -1]
- oriented_gauss_code()[source]¶
Return the oriented Gauss code of
self.The oriented Gauss code has two parts:
the Gauss code
the orientation of each crossing
The following orientation was taken into consideration for construction of knots:
From the outgoing of the overcrossing if we move in the clockwise direction to reach the outgoing of the undercrossing then we label that crossing as \(-1\).
From the outgoing of the overcrossing if we move in the anticlockwise direction to reach the outgoing of the undercrossing then we label that crossing as \(+1\).
One more consideration we take in while constructing the orientation is the order of the orientation is same as the ordering of the crossings in the Gauss code.
Note
Convention: under is denoted by \(-1\), and over by \(+1\) in the crossing info.
EXAMPLES:
sage: L = Link([[1, 10, 2, 11], [6, 3, 7, 2], [3, 9, 4, 12], ....: [9, 6, 10, 5], [8, 4, 5, 1], [11, 7, 12, 8]]) sage: L.oriented_gauss_code() [[[-1, 2, -3, 5], [4, -2, 6, -5], [-4, 1, -6, 3]], [-1, 1, 1, 1, -1, -1]] sage: L = Link([[1, 3, 2, 4], [6, 2, 3, 1], [7, 5, 8, 4], [5, 7, 6, 8]]) sage: L.oriented_gauss_code() [[[-1, 2], [-3, 4], [1, 3, -4, -2]], [-1, -1, 1, 1]] sage: B = BraidGroup(8) sage: b = B([1, 1, 1, 1, 1]) sage: L = Link(b) sage: L.oriented_gauss_code() [[[1, -2, 3, -4, 5, -1, 2, -3, 4, -5]], [1, 1, 1, 1, 1]]
>>> from sage.all import * >>> L = Link([[Integer(1), Integer(10), Integer(2), Integer(11)], [Integer(6), Integer(3), Integer(7), Integer(2)], [Integer(3), Integer(9), Integer(4), Integer(12)], ... [Integer(9), Integer(6), Integer(10), Integer(5)], [Integer(8), Integer(4), Integer(5), Integer(1)], [Integer(11), Integer(7), Integer(12), Integer(8)]]) >>> L.oriented_gauss_code() [[[-1, 2, -3, 5], [4, -2, 6, -5], [-4, 1, -6, 3]], [-1, 1, 1, 1, -1, -1]] >>> L = Link([[Integer(1), Integer(3), Integer(2), Integer(4)], [Integer(6), Integer(2), Integer(3), Integer(1)], [Integer(7), Integer(5), Integer(8), Integer(4)], [Integer(5), Integer(7), Integer(6), Integer(8)]]) >>> L.oriented_gauss_code() [[[-1, 2], [-3, 4], [1, 3, -4, -2]], [-1, -1, 1, 1]] >>> B = BraidGroup(Integer(8)) >>> b = B([Integer(1), Integer(1), Integer(1), Integer(1), Integer(1)]) >>> L = Link(b) >>> L.oriented_gauss_code() [[[1, -2, 3, -4, 5, -1, 2, -3, 4, -5]], [1, 1, 1, 1, 1]]
- pd_code()[source]¶
Return the planar diagram code of
self.The planar diagram is returned in the following format.
We construct the crossing by starting with the entering component of the undercrossing, move in the anti-clockwise direction (see the note below) and then generate the list. If the crossing is given by \([a, b, c, d]\), then we interpret this information as:
\(a\) is the entering component of the undercrossing;
\(b, d\) are the components of the overcrossing;
\(c\) is the leaving component of the undercrossing.
Note
Until version 10.0 the convention to read the
PDcode has been to list the components in clockwise direction. As of version 10.1 the convention has changed, since it was opposite to the usage in most other places.Thus, if you use
PDcodes from former Sage releases with this version you should check for the correct mirror type.EXAMPLES:
sage: L = Link([[[1, -2, 3, -4, 2, -1, 4, -3]], [1, 1, -1, -1]]) sage: L.pd_code() [[6, 2, 7, 1], [2, 6, 3, 5], [8, 3, 1, 4], [4, 7, 5, 8]] sage: B = BraidGroup(2) sage: b = B([1, 1, 1, 1, 1]) sage: L = Link(b) sage: L.pd_code() [[2, 4, 3, 1], [4, 6, 5, 3], [6, 8, 7, 5], [8, 10, 9, 7], [10, 2, 1, 9]] sage: L = Link([[[2, -1], [1, -2]], [1, 1]]) sage: L.pd_code() [[2, 4, 1, 3], [4, 2, 3, 1]] sage: L = Link([[1, 2, 3, 3], [2, 4, 5, 5], [4, 1, 7, 7]]) sage: L.pd_code() [[1, 2, 3, 3], [2, 4, 5, 5], [4, 1, 7, 7]]
>>> from sage.all import * >>> L = Link([[[Integer(1), -Integer(2), Integer(3), -Integer(4), Integer(2), -Integer(1), Integer(4), -Integer(3)]], [Integer(1), Integer(1), -Integer(1), -Integer(1)]]) >>> L.pd_code() [[6, 2, 7, 1], [2, 6, 3, 5], [8, 3, 1, 4], [4, 7, 5, 8]] >>> B = BraidGroup(Integer(2)) >>> b = B([Integer(1), Integer(1), Integer(1), Integer(1), Integer(1)]) >>> L = Link(b) >>> L.pd_code() [[2, 4, 3, 1], [4, 6, 5, 3], [6, 8, 7, 5], [8, 10, 9, 7], [10, 2, 1, 9]] >>> L = Link([[[Integer(2), -Integer(1)], [Integer(1), -Integer(2)]], [Integer(1), Integer(1)]]) >>> L.pd_code() [[2, 4, 1, 3], [4, 2, 3, 1]] >>> L = Link([[Integer(1), Integer(2), Integer(3), Integer(3)], [Integer(2), Integer(4), Integer(5), Integer(5)], [Integer(4), Integer(1), Integer(7), Integer(7)]]) >>> L.pd_code() [[1, 2, 3, 3], [2, 4, 5, 5], [4, 1, 7, 7]]
- plot(gap=0.1, component_gap=0.5, solver=None, color='blue', **kwargs)[source]¶
Plot
self.INPUT:
gap– (default: 0.1) the size of the blank gap left for the crossingscomponent_gap– (default: 0.5) the gap between isolated componentssolver– the linear solver to use, seeMixedIntegerLinearProgramcolor– string (default:'blue'); a color or a coloring (as returned bycolorings()
The usual keywords for plots can be used here too.
EXAMPLES:
We construct the simplest version of the unknot:
sage: L = Link([[2, 1, 1, 2]]) sage: L.plot() # needs sage.plot Graphics object consisting of ... graphics primitives
>>> from sage.all import * >>> L = Link([[Integer(2), Integer(1), Integer(1), Integer(2)]]) >>> L.plot() # needs sage.plot Graphics object consisting of ... graphics primitives
We construct a more interesting example of the unknot:
sage: L = Link([[2, 1, 4, 5], [3, 5, 6, 7], [4, 1, 9, 6], [9, 2, 3, 7]]) sage: L.plot() # needs sage.plot Graphics object consisting of ... graphics primitives
>>> from sage.all import * >>> L = Link([[Integer(2), Integer(1), Integer(4), Integer(5)], [Integer(3), Integer(5), Integer(6), Integer(7)], [Integer(4), Integer(1), Integer(9), Integer(6)], [Integer(9), Integer(2), Integer(3), Integer(7)]]) >>> L.plot() # needs sage.plot Graphics object consisting of ... graphics primitives
The “monster” unknot:
sage: L = Link([[3,1,2,4], [8,9,1,7], [5,6,7,3], [4,18,6,5], ....: [17,19,8,18], [9,10,11,14], [10,12,13,11], ....: [12,19,15,13], [20,16,14,15], [16,20,17,2]]) sage: L.plot() # needs sage.plot Graphics object consisting of ... graphics primitives
>>> from sage.all import * >>> L = Link([[Integer(3),Integer(1),Integer(2),Integer(4)], [Integer(8),Integer(9),Integer(1),Integer(7)], [Integer(5),Integer(6),Integer(7),Integer(3)], [Integer(4),Integer(18),Integer(6),Integer(5)], ... [Integer(17),Integer(19),Integer(8),Integer(18)], [Integer(9),Integer(10),Integer(11),Integer(14)], [Integer(10),Integer(12),Integer(13),Integer(11)], ... [Integer(12),Integer(19),Integer(15),Integer(13)], [Integer(20),Integer(16),Integer(14),Integer(15)], [Integer(16),Integer(20),Integer(17),Integer(2)]]) >>> L.plot() # needs sage.plot Graphics object consisting of ... graphics primitives
The Ochiai unknot:
sage: L = Link([[[1,-2,-3,-8,-12,13,-14,15,-7,-1,2,-4,10,11,-13,12, ....: -11,-16,4,3,-5,6,-9,7,-15,14,16,-10,8,9,-6,5]], ....: [-1,-1,1,1,1,1,-1,1,1,-1,1,-1,-1,-1,-1,-1]]) sage: L.plot() # needs sage.plot Graphics object consisting of ... graphics primitives
>>> from sage.all import * >>> L = Link([[[Integer(1),-Integer(2),-Integer(3),-Integer(8),-Integer(12),Integer(13),-Integer(14),Integer(15),-Integer(7),-Integer(1),Integer(2),-Integer(4),Integer(10),Integer(11),-Integer(13),Integer(12), ... -Integer(11),-Integer(16),Integer(4),Integer(3),-Integer(5),Integer(6),-Integer(9),Integer(7),-Integer(15),Integer(14),Integer(16),-Integer(10),Integer(8),Integer(9),-Integer(6),Integer(5)]], ... [-Integer(1),-Integer(1),Integer(1),Integer(1),Integer(1),Integer(1),-Integer(1),Integer(1),Integer(1),-Integer(1),Integer(1),-Integer(1),-Integer(1),-Integer(1),-Integer(1),-Integer(1)]]) >>> L.plot() # needs sage.plot Graphics object consisting of ... graphics primitives
One of the representations of the trefoil knot:
sage: L = Link([[1, 5, 2, 4], [5, 3, 6, 2], [3, 1, 4, 6]]) sage: L.plot() # needs sage.plot Graphics object consisting of 14 graphics primitives
>>> from sage.all import * >>> L = Link([[Integer(1), Integer(5), Integer(2), Integer(4)], [Integer(5), Integer(3), Integer(6), Integer(2)], [Integer(3), Integer(1), Integer(4), Integer(6)]]) >>> L.plot() # needs sage.plot Graphics object consisting of 14 graphics primitives
The figure-eight knot:
sage: L = Link([[2, 1, 4, 5], [5, 6, 7, 3], [6, 4, 1, 9], [9, 2, 3, 7]]) sage: L.plot() # needs sage.plot Graphics object consisting of ... graphics primitives
>>> from sage.all import * >>> L = Link([[Integer(2), Integer(1), Integer(4), Integer(5)], [Integer(5), Integer(6), Integer(7), Integer(3)], [Integer(6), Integer(4), Integer(1), Integer(9)], [Integer(9), Integer(2), Integer(3), Integer(7)]]) >>> L.plot() # needs sage.plot Graphics object consisting of ... graphics primitives
The knot \(K11n121\) in [KnotAtlas]:
sage: L = Link([[4,2,5,1], [10,3,11,4], [5,16,6,17], [7,12,8,13], ....: [18,9,19,10], [2,11,3,12], [13,20,14,21], [15,6,16,7], ....: [22,18,1,17], [8,19,9,20], [21,14,22,15]]) sage: L.plot() # needs sage.plot Graphics object consisting of ... graphics primitives
>>> from sage.all import * >>> L = Link([[Integer(4),Integer(2),Integer(5),Integer(1)], [Integer(10),Integer(3),Integer(11),Integer(4)], [Integer(5),Integer(16),Integer(6),Integer(17)], [Integer(7),Integer(12),Integer(8),Integer(13)], ... [Integer(18),Integer(9),Integer(19),Integer(10)], [Integer(2),Integer(11),Integer(3),Integer(12)], [Integer(13),Integer(20),Integer(14),Integer(21)], [Integer(15),Integer(6),Integer(16),Integer(7)], ... [Integer(22),Integer(18),Integer(1),Integer(17)], [Integer(8),Integer(19),Integer(9),Integer(20)], [Integer(21),Integer(14),Integer(22),Integer(15)]]) >>> L.plot() # needs sage.plot Graphics object consisting of ... graphics primitives
One of the representations of the Hopf link:
sage: L = Link([[1, 4, 2, 3], [4, 1, 3, 2]]) sage: L.plot() # needs sage.plot Graphics object consisting of ... graphics primitives
>>> from sage.all import * >>> L = Link([[Integer(1), Integer(4), Integer(2), Integer(3)], [Integer(4), Integer(1), Integer(3), Integer(2)]]) >>> L.plot() # needs sage.plot Graphics object consisting of ... graphics primitives
Plotting links with multiple isolated components:
sage: L = Link([[[-1, 2, -3, 1, -2, 3], [4, -5, 6, -4, 5, -6]], ....: [1, 1, 1, 1, 1, 1]]) sage: L.plot() # needs sage.plot Graphics object consisting of ... graphics primitives
>>> from sage.all import * >>> L = Link([[[-Integer(1), Integer(2), -Integer(3), Integer(1), -Integer(2), Integer(3)], [Integer(4), -Integer(5), Integer(6), -Integer(4), Integer(5), -Integer(6)]], ... [Integer(1), Integer(1), Integer(1), Integer(1), Integer(1), Integer(1)]]) >>> L.plot() # needs sage.plot Graphics object consisting of ... graphics primitives
If a coloring is passed, the different arcs are plotted with the corresponding colors (see
colorings()):sage: B = BraidGroup(4) sage: b = B([1,2,3,1,2,-1,-3,2,3]) sage: L = Link(b) sage: L.plot(color=L.colorings()[0]) # needs sage.plot Graphics object consisting of ... graphics primitives
>>> from sage.all import * >>> B = BraidGroup(Integer(4)) >>> b = B([Integer(1),Integer(2),Integer(3),Integer(1),Integer(2),-Integer(1),-Integer(3),Integer(2),Integer(3)]) >>> L = Link(b) >>> L.plot(color=L.colorings()[Integer(0)]) # needs sage.plot Graphics object consisting of ... graphics primitives
- regions()[source]¶
Return the regions from the link diagram of
self.Regions are obtained always turning left at each crossing.
Then the regions are represented as a list with the segments that form its boundary, with a sign depending on the orientation of the segment as part of the boundary.
EXAMPLES:
sage: L = Link([[[-1, +2, -3, 4, +5, +1, -2, +6, +7, 3, -4, -7, -6,-5]], ....: [-1, -1, -1, -1, 1, -1, 1]]) sage: L.regions() [[14, -5, 12, -9], [13, 9], [11, 5, 1, 7, 3], [10, -3, 8, -13], [6, -1], [4, -11], [2, -7], [-2, -6, -14, -8], [-4, -10, -12]] sage: L = Link([[[1, -2, 3, -4, 2, -1, 4, -3]],[1, 1, -1, -1]]) sage: L.regions() [[8, 4], [7, -4, 1], [6, -1, -3], [5, 3, -8], [2, -5, -7], [-2, -6]] sage: L = Link([[[-1, +2, 3, -4, 5, -6, 7, 8, -2, -5, +6, +1, -8, -3, 4, -7]], ....: [-1, -1, -1, -1, 1, 1, -1, 1]]) sage: L.regions() [[16, 8, 14, 4], [15, -4], [13, -8, 1], [12, -1, -7], [11, 7, -16, 5], [10, -5, -15, -3], [9, 3, -14], [6, -11], [2, -9, -13], [-2, -12, -6, -10]] sage: B = BraidGroup(2) sage: L = Link(B([-1, -1, -1])) sage: L.regions() [[6, -5], [5, 1, 3], [4, -3], [2, -1], [-2, -6, -4]] sage: L = Link([[[1, -2, 3, -4], [-1, 5, -3, 2, -5, 4]], ....: [-1, 1, 1, -1, -1]]) sage: L.regions() [[10, -4, -7], [9, 7, -3], [8, 3], [6, -9, -2], [5, 2, -8, 4], [1, -5], [-1, -10, -6]] sage: L = Link([[1, 3, 3, 2], [2, 4, 4, 5], [5, 6, 6, 7], [7, 8, 8, 1]]) sage: L.regions() [[-3], [-4], [-6], [-8], [7, 1, 2, 5], [-1, 8, -7, 6, -5, 4, -2, 3]]
>>> from sage.all import * >>> L = Link([[[-Integer(1), +Integer(2), -Integer(3), Integer(4), +Integer(5), +Integer(1), -Integer(2), +Integer(6), +Integer(7), Integer(3), -Integer(4), -Integer(7), -Integer(6),-Integer(5)]], ... [-Integer(1), -Integer(1), -Integer(1), -Integer(1), Integer(1), -Integer(1), Integer(1)]]) >>> L.regions() [[14, -5, 12, -9], [13, 9], [11, 5, 1, 7, 3], [10, -3, 8, -13], [6, -1], [4, -11], [2, -7], [-2, -6, -14, -8], [-4, -10, -12]] >>> L = Link([[[Integer(1), -Integer(2), Integer(3), -Integer(4), Integer(2), -Integer(1), Integer(4), -Integer(3)]],[Integer(1), Integer(1), -Integer(1), -Integer(1)]]) >>> L.regions() [[8, 4], [7, -4, 1], [6, -1, -3], [5, 3, -8], [2, -5, -7], [-2, -6]] >>> L = Link([[[-Integer(1), +Integer(2), Integer(3), -Integer(4), Integer(5), -Integer(6), Integer(7), Integer(8), -Integer(2), -Integer(5), +Integer(6), +Integer(1), -Integer(8), -Integer(3), Integer(4), -Integer(7)]], ... [-Integer(1), -Integer(1), -Integer(1), -Integer(1), Integer(1), Integer(1), -Integer(1), Integer(1)]]) >>> L.regions() [[16, 8, 14, 4], [15, -4], [13, -8, 1], [12, -1, -7], [11, 7, -16, 5], [10, -5, -15, -3], [9, 3, -14], [6, -11], [2, -9, -13], [-2, -12, -6, -10]] >>> B = BraidGroup(Integer(2)) >>> L = Link(B([-Integer(1), -Integer(1), -Integer(1)])) >>> L.regions() [[6, -5], [5, 1, 3], [4, -3], [2, -1], [-2, -6, -4]] >>> L = Link([[[Integer(1), -Integer(2), Integer(3), -Integer(4)], [-Integer(1), Integer(5), -Integer(3), Integer(2), -Integer(5), Integer(4)]], ... [-Integer(1), Integer(1), Integer(1), -Integer(1), -Integer(1)]]) >>> L.regions() [[10, -4, -7], [9, 7, -3], [8, 3], [6, -9, -2], [5, 2, -8, 4], [1, -5], [-1, -10, -6]] >>> L = Link([[Integer(1), Integer(3), Integer(3), Integer(2)], [Integer(2), Integer(4), Integer(4), Integer(5)], [Integer(5), Integer(6), Integer(6), Integer(7)], [Integer(7), Integer(8), Integer(8), Integer(1)]]) >>> L.regions() [[-3], [-4], [-6], [-8], [7, 1, 2, 5], [-1, 8, -7, 6, -5, 4, -2, 3]]
Note
The link diagram is assumed to have only one completely isolated component. This is because otherwise some regions would have disconnected boundary.
- remove_loops()[source]¶
Return an ambient isotopic link in which all loops are removed.
EXAMPLES:
sage: b = BraidGroup(4)((3, 2, -1, -1)) sage: L = Link(b) sage: L.remove_loops() Link with 2 components represented by 2 crossings sage: K4 = Link([[1, 7, 2, 6], [3, 1, 4, 8], [5, 5, 6, 4], [7, 3, 8, 2]]) sage: K3 = K4.remove_loops() sage: K3.pd_code() [[1, 7, 2, 4], [3, 1, 4, 8], [7, 3, 8, 2]] sage: U = Link([[1, 2, 2, 1]]) sage: U.remove_loops() Link with 1 component represented by 0 crossings
>>> from sage.all import * >>> b = BraidGroup(Integer(4))((Integer(3), Integer(2), -Integer(1), -Integer(1))) >>> L = Link(b) >>> L.remove_loops() Link with 2 components represented by 2 crossings >>> K4 = Link([[Integer(1), Integer(7), Integer(2), Integer(6)], [Integer(3), Integer(1), Integer(4), Integer(8)], [Integer(5), Integer(5), Integer(6), Integer(4)], [Integer(7), Integer(3), Integer(8), Integer(2)]]) >>> K3 = K4.remove_loops() >>> K3.pd_code() [[1, 7, 2, 4], [3, 1, 4, 8], [7, 3, 8, 2]] >>> U = Link([[Integer(1), Integer(2), Integer(2), Integer(1)]]) >>> U.remove_loops() Link with 1 component represented by 0 crossings
- reverse()[source]¶
Return the reverse of
self. This is the link obtained fromselfby reverting the orientation on all components.EXAMPLES:
sage: K3 = Knot([[5, 2, 4, 1], [3, 6, 2, 5], [1, 4, 6, 3]]) sage: K3r = K3.reverse(); K3r.pd_code() [[4, 1, 5, 2], [2, 5, 3, 6], [6, 3, 1, 4]] sage: K3 == K3r True
>>> from sage.all import * >>> K3 = Knot([[Integer(5), Integer(2), Integer(4), Integer(1)], [Integer(3), Integer(6), Integer(2), Integer(5)], [Integer(1), Integer(4), Integer(6), Integer(3)]]) >>> K3r = K3.reverse(); K3r.pd_code() [[4, 1, 5, 2], [2, 5, 3, 6], [6, 3, 1, 4]] >>> K3 == K3r True
a non reversable knot:
sage: K8_17 = Knot([[6, 1, 7, 2], [14, 7, 15, 8], [8, 4, 9, 3], ....: [2, 14, 3, 13], [12, 6, 13, 5], [4, 10, 5, 9], ....: [16, 11, 1, 12], [10, 15, 11, 16]]) sage: K8_17r = K8_17.reverse() sage: b = K8_17.braid(); b s0^2*s1^-1*(s1^-1*s0)^2*s1^-1 sage: br = K8_17r.braid(); br s0^-1*s1*s0^-2*s1^2*s0^-1*s1 sage: b.is_conjugated(br) False sage: b == br.reverse() False sage: b.is_conjugated(br.reverse()) True sage: K8_17b = Link(b) sage: K8_17br = K8_17b.reverse() sage: bbr = K8_17br.braid(); bbr (s1^-1*s0)^2*s1^-2*s0^2 sage: br == bbr False sage: br.is_conjugated(bbr) True
>>> from sage.all import * >>> K8_17 = Knot([[Integer(6), Integer(1), Integer(7), Integer(2)], [Integer(14), Integer(7), Integer(15), Integer(8)], [Integer(8), Integer(4), Integer(9), Integer(3)], ... [Integer(2), Integer(14), Integer(3), Integer(13)], [Integer(12), Integer(6), Integer(13), Integer(5)], [Integer(4), Integer(10), Integer(5), Integer(9)], ... [Integer(16), Integer(11), Integer(1), Integer(12)], [Integer(10), Integer(15), Integer(11), Integer(16)]]) >>> K8_17r = K8_17.reverse() >>> b = K8_17.braid(); b s0^2*s1^-1*(s1^-1*s0)^2*s1^-1 >>> br = K8_17r.braid(); br s0^-1*s1*s0^-2*s1^2*s0^-1*s1 >>> b.is_conjugated(br) False >>> b == br.reverse() False >>> b.is_conjugated(br.reverse()) True >>> K8_17b = Link(b) >>> K8_17br = K8_17b.reverse() >>> bbr = K8_17br.braid(); bbr (s1^-1*s0)^2*s1^-2*s0^2 >>> br == bbr False >>> br.is_conjugated(bbr) True
- seifert_circles()[source]¶
Return the Seifert circles from the link diagram of
self.Seifert circles are the circles obtained by smoothing all crossings respecting the orientation of the segments.
Each Seifert circle is represented as a list of the segments that form it.
EXAMPLES:
sage: L = Link([[[1, -2, 3, -4, 2, -1, 4, -3]], [1, 1, -1, -1]]) sage: L.seifert_circles() [[1, 7, 5, 3], [2, 6], [4, 8]] sage: L = Link([[[-1, 2, 3, -4, 5, -6, 7, 8, -2, -5, 6, 1, -8, -3, 4, -7]], ....: [-1, -1, -1, -1, 1, 1, -1, 1]]) sage: L.seifert_circles() [[1, 13, 9, 3, 15, 5, 11, 7], [2, 10, 6, 12], [4, 16, 8, 14]] sage: L = Link([[[-1, 2, -3, 4, 5, 1, -2, 6, 7, 3, -4, -7, -6, -5]], ....: [-1, -1, -1, -1, 1, -1, 1]]) sage: L.seifert_circles() [[1, 7, 3, 11, 5], [2, 8, 14, 6], [4, 12, 10], [9, 13]] sage: L = Link([[1, 7, 2, 6], [7, 3, 8, 2], [3, 11, 4, 10], [11, 5, 12, 4], ....: [14, 5, 1, 6], [13, 9, 14, 8], [12, 9, 13, 10]]) sage: L.seifert_circles() [[1, 7, 3, 11, 5], [2, 8, 14, 6], [4, 12, 10], [9, 13]] sage: L = Link([[[-1, 2, -3, 5], [4, -2, 6, -5], [-4, 1, -6, 3]], ....: [-1, 1, 1, 1, -1, -1]]) sage: L.seifert_circles() [[1, 11, 8], [2, 7, 12, 4, 5, 10], [3, 9, 6]] sage: B = BraidGroup(2) sage: L = Link(B([1, 1, 1])) sage: L.seifert_circles() [[1, 3, 5], [2, 4, 6]]
>>> from sage.all import * >>> L = Link([[[Integer(1), -Integer(2), Integer(3), -Integer(4), Integer(2), -Integer(1), Integer(4), -Integer(3)]], [Integer(1), Integer(1), -Integer(1), -Integer(1)]]) >>> L.seifert_circles() [[1, 7, 5, 3], [2, 6], [4, 8]] >>> L = Link([[[-Integer(1), Integer(2), Integer(3), -Integer(4), Integer(5), -Integer(6), Integer(7), Integer(8), -Integer(2), -Integer(5), Integer(6), Integer(1), -Integer(8), -Integer(3), Integer(4), -Integer(7)]], ... [-Integer(1), -Integer(1), -Integer(1), -Integer(1), Integer(1), Integer(1), -Integer(1), Integer(1)]]) >>> L.seifert_circles() [[1, 13, 9, 3, 15, 5, 11, 7], [2, 10, 6, 12], [4, 16, 8, 14]] >>> L = Link([[[-Integer(1), Integer(2), -Integer(3), Integer(4), Integer(5), Integer(1), -Integer(2), Integer(6), Integer(7), Integer(3), -Integer(4), -Integer(7), -Integer(6), -Integer(5)]], ... [-Integer(1), -Integer(1), -Integer(1), -Integer(1), Integer(1), -Integer(1), Integer(1)]]) >>> L.seifert_circles() [[1, 7, 3, 11, 5], [2, 8, 14, 6], [4, 12, 10], [9, 13]] >>> L = Link([[Integer(1), Integer(7), Integer(2), Integer(6)], [Integer(7), Integer(3), Integer(8), Integer(2)], [Integer(3), Integer(11), Integer(4), Integer(10)], [Integer(11), Integer(5), Integer(12), Integer(4)], ... [Integer(14), Integer(5), Integer(1), Integer(6)], [Integer(13), Integer(9), Integer(14), Integer(8)], [Integer(12), Integer(9), Integer(13), Integer(10)]]) >>> L.seifert_circles() [[1, 7, 3, 11, 5], [2, 8, 14, 6], [4, 12, 10], [9, 13]] >>> L = Link([[[-Integer(1), Integer(2), -Integer(3), Integer(5)], [Integer(4), -Integer(2), Integer(6), -Integer(5)], [-Integer(4), Integer(1), -Integer(6), Integer(3)]], ... [-Integer(1), Integer(1), Integer(1), Integer(1), -Integer(1), -Integer(1)]]) >>> L.seifert_circles() [[1, 11, 8], [2, 7, 12, 4, 5, 10], [3, 9, 6]] >>> B = BraidGroup(Integer(2)) >>> L = Link(B([Integer(1), Integer(1), Integer(1)])) >>> L.seifert_circles() [[1, 3, 5], [2, 4, 6]]
- seifert_matrix()[source]¶
Return the Seifert matrix associated with
self.ALGORITHM:
This is the algorithm presented in Section 3.3 of [Col2013].
OUTPUT:
The intersection matrix of a (not necessarily minimal) Seifert surface.
EXAMPLES:
sage: # needs sage.modules sage: B = BraidGroup(4) sage: L = Link(B([-1, 3, 1, 3])) sage: L.seifert_matrix() [ 0 0] [ 0 -1] sage: B = BraidGroup(8) sage: L = Link(B([-1, 3, 1, 5, 1, 7, 1, 6])) sage: L.seifert_matrix() [ 0 0 0] [ 1 -1 0] [ 0 1 -1] sage: L = Link(B([-2, 4, 1, 6, 1, 4])) sage: L.seifert_matrix() [-1 0] [ 0 -1]
>>> from sage.all import * >>> # needs sage.modules >>> B = BraidGroup(Integer(4)) >>> L = Link(B([-Integer(1), Integer(3), Integer(1), Integer(3)])) >>> L.seifert_matrix() [ 0 0] [ 0 -1] >>> B = BraidGroup(Integer(8)) >>> L = Link(B([-Integer(1), Integer(3), Integer(1), Integer(5), Integer(1), Integer(7), Integer(1), Integer(6)])) >>> L.seifert_matrix() [ 0 0 0] [ 1 -1 0] [ 0 1 -1] >>> L = Link(B([-Integer(2), Integer(4), Integer(1), Integer(6), Integer(1), Integer(4)])) >>> L.seifert_matrix() [-1 0] [ 0 -1]
- signature()[source]¶
Return the signature of
self.This is defined as the signature of the symmetric matrix
\[V + V^{t},\]where \(V\) is the
Seifert matrix.See also
EXAMPLES:
sage: # needs sage.modules sage: B = BraidGroup(4) sage: L = Link(B([-1, 3, 1, 3])) sage: L.signature() -1 sage: B = BraidGroup(8) sage: L = Link(B([-2, 4, 1, 6, 1, 4])) sage: L.signature() -2 sage: L = Link(B([1, 2, 1, 2])) sage: L.signature() -2
>>> from sage.all import * >>> # needs sage.modules >>> B = BraidGroup(Integer(4)) >>> L = Link(B([-Integer(1), Integer(3), Integer(1), Integer(3)])) >>> L.signature() -1 >>> B = BraidGroup(Integer(8)) >>> L = Link(B([-Integer(2), Integer(4), Integer(1), Integer(6), Integer(1), Integer(4)])) >>> L.signature() -2 >>> L = Link(B([Integer(1), Integer(2), Integer(1), Integer(2)])) >>> L.signature() -2
- writhe()[source]¶
Return the writhe of
self.EXAMPLES:
sage: L = Link([[[1, -2, 3, -4, 2, -1, 4, -3]],[1, 1, -1, -1]]) sage: L.writhe() 0 sage: L = Link([[[-1, 2, -3, 4, 5, 1, -2, 6, 7, 3, -4, -7, -6,-5]], ....: [-1, -1, -1, -1, 1, -1, 1]]) sage: L.writhe() -3 sage: L = Link([[[-1, 2, 3, -4, 5, -6, 7, 8, -2, -5, 6, 1, -8, -3, 4, -7]], ....: [-1, -1, -1, -1, 1, 1, -1, 1]]) sage: L.writhe() -2
>>> from sage.all import * >>> L = Link([[[Integer(1), -Integer(2), Integer(3), -Integer(4), Integer(2), -Integer(1), Integer(4), -Integer(3)]],[Integer(1), Integer(1), -Integer(1), -Integer(1)]]) >>> L.writhe() 0 >>> L = Link([[[-Integer(1), Integer(2), -Integer(3), Integer(4), Integer(5), Integer(1), -Integer(2), Integer(6), Integer(7), Integer(3), -Integer(4), -Integer(7), -Integer(6),-Integer(5)]], ... [-Integer(1), -Integer(1), -Integer(1), -Integer(1), Integer(1), -Integer(1), Integer(1)]]) >>> L.writhe() -3 >>> L = Link([[[-Integer(1), Integer(2), Integer(3), -Integer(4), Integer(5), -Integer(6), Integer(7), Integer(8), -Integer(2), -Integer(5), Integer(6), Integer(1), -Integer(8), -Integer(3), Integer(4), -Integer(7)]], ... [-Integer(1), -Integer(1), -Integer(1), -Integer(1), Integer(1), Integer(1), -Integer(1), Integer(1)]]) >>> L.writhe() -2