Knots#
AUTHORS:
Miguel Angel Marco Buzunariz
Amit Jamadagni
 class sage.knots.knot.Knot(data, check=True)#

A knot.
A knot is defined as embedding of the circle \(\mathbb{S}^1\) in the 3dimensional 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.
See also
INPUT:
data
– seeLink
for the allowable inputscheck
– optional, defaultTrue
. IfTrue
, make sure that the data define a knot, not a link
EXAMPLES:
We construct the knot \(8_{14}\) and compute some invariants:
sage: B = BraidGroup(4) sage: K = Knot(B([1,1,1,2,1,2,3,2,3]))
sage: K.alexander_polynomial() 2*t^2 + 8*t^1  11 + 8*t  2*t^2 sage: K.jones_polynomial() t^7  3*t^6 + 4*t^5  5*t^4 + 6*t^3  5*t^2 + 4*t + 1/t  2 sage: K.determinant() 31 sage: K.signature() 2 sage: K.colored_jones_polynomial(2) # long time q^1  2 + 4*q  5*q^2 + 6*q^3  5*q^4 + 4*q^5  3*q^6 + q^7
REFERENCES:
 arf_invariant()#
Return the Arf invariant.
EXAMPLES:
sage: B = BraidGroup(4) sage: K = Knot(B([1, 2, 1, 2])) sage: K.arf_invariant() 0 sage: B = BraidGroup(8) sage: K = Knot(B([2, 3, 1, 2, 1, 4])) sage: K.arf_invariant() 0 sage: K = Knot(B([1, 2, 1, 2])) sage: K.arf_invariant() 1
 colored_jones_polynomial(N, variab=None, try_inverse=True)#
Return the colored Jones polynomial of the trace closure of the braid.
INPUT:
N
– integer; the number of colorsvariab
– (default: \(q\)) the variable in the resulting Laurent polynomialtry_inverse
– boolean (default:True
); ifTrue
, attempt a faster calculation by using the inverse of the braid
ALGORITHM:
The algorithm used is described in [HL2018] for the corresponding braid representation. We follow their notation, but work in a suitable free algebra over a Laurent polynomial ring in one variable to simplify bookkeeping.
EXAMPLES:
sage: W = Knots() sage: K = W.from_dowker_code([4,6,2]) sage: K.colored_jones_polynomial(2) q^4 + q^3 + q^1 sage: K.colored_jones_polynomial(2, 't') t^4 + t^3 + t^1 sage: R.<t> = LaurentPolynomialRing(ZZ) sage: K.colored_jones_polynomial(2, t) t^4  t^3  t^1 sage: R.<t> = ZZ[] sage: K.colored_jones_polynomial(2, t+1) (t^3 + 3*t^2 + 4*t + 1)/(t^4 + 4*t^3 + 6*t^2 + 4*t + 1)
 connected_sum(other)#
Return the oriented connected sum of
self
andother
.Note
We give the knots an orientation based upon the braid representation.
INPUT:
other
– a knot
OUTPUT:
A knot equivalent to the connected sum of
self
andother
.EXAMPLES:
sage: B = BraidGroup(2) sage: trefoil = Knot(B([1,1,1])) sage: K = trefoil.connected_sum(trefoil); K Knot represented by 6 crossings sage: K.braid() s0^3*s1^1*s0^3*s1
sage: rev_trefoil = Knot(B([1,1,1])) sage: K2 = trefoil.connected_sum(rev_trefoil); K2 Knot represented by 6 crossings sage: K2.braid() s0^3*s1^1*s0^3*s1
Observe that both knots have according
dowker_notation
(showing that the constructing from DTcode may not be unique for non prime knots, seefrom_dowker_code()
):sage: K.dowker_notation() [(4, 1), (2, 5), (6, 3), (10, 7), (8, 11), (12, 9)] sage: K2.dowker_notation() [(4, 1), (2, 5), (6, 3), (7, 10), (11, 8), (9, 12)] sage: K.homfly_polynomial() == K2.homfly_polynomial() False
REFERENCES:
 dt_code()#
Return the DT code of
self
.ALGORITHM:
The DT code is generated by the following way:
Start moving along the knot, as we encounter the crossings we start numbering them, so every crossing has two numbers assigned to it once we have traced the entire knot. Now we take the even number associated with every crossing.
The following sign convention is to be followed:
Take the even number with a negative sign if it is an overcrossing that we are encountering.
OUTPUT: DT code representation of the knot
EXAMPLES:
sage: K = Knot([[1,5,2,4],[5,3,6,2],[3,1,4,6]]) sage: K.dt_code() [4, 6, 2] sage: B = BraidGroup(4) sage: K = Knot(B([1, 2, 1, 2])) sage: K.dt_code() [4, 6, 8, 2] sage: K = Knot([[[1, 2, 3, 4, 5, 1, 2, 3, 4, 5]], ....: [1, 1, 1, 1, 1]]) sage: K.dt_code() [6, 8, 10, 2, 4]
 class sage.knots.knot.Knots#

The set for all knots, as a monoid for the connected sum.
 an_element()#
Return the trefoil knot.
EXAMPLES:
sage: Knots().an_element() Knot represented by 3 crossings
 from_dowker_code(code)#
Build a knot from a DowkerThistlethwaite code.
The DowkerThistlethwaite code of a knot diagram is defined as follows.
Start following the knot diagram at some regular point. Label the crossings by a number (starting from number 1) in the order in which they are met. At the end, every crossing gets numbered twice, once by an even number and once by an odd number. When meeting an overcrossing with even number, use instead the negative of this even number as label.
Then the set of crossings gives a set of pairs (odd, even). Sort this set according to the odd component, and then keep only the even components in the same order. This is the DowkerThistlethwaite code.
INPUT:
a list of signed even numbers, the DowkerThistlethwaite code of a knot
OUTPUT:
a knot
Warning
In general the DowkerThistlethwaite code does not describe a knot uniquely. It is not only insensitive on mirror images, but may also mix up non prime knots. For example
[4, 6, 2, 10, 12, 8]
describes the connected sum of two trefoil knots, as well as the connected sum of a trefoil with its mirror (see the corresponding example in the documentation ofconnected_sum()
).EXAMPLES:
sage: W = Knots() sage: K1 = W.from_dowker_code([8,10,2,12,4,6]) sage: K1.dowker_notation() [(5, 2), (9, 4), (11, 6), (1, 8), (3, 10), (7, 12)] sage: W.from_dowker_code([6,10,14,12,16,2,18,4,8]) Knot represented by 9 crossings sage: W.from_dowker_code([4,8,10,14,2,16,18,6,12]) Knot represented by 9 crossings sage: K3 = W.from_dowker_code([6,12,2,8,4,10]); K3 Knot represented by 6 crossings sage: K3.dowker_notation() [(5, 2), (4, 9), (1, 6), (7, 8), (10, 11), (12, 3)]
See also
dowker_notation()
REFERENCES:
 from_gauss_code(gauss)#
Build a knot from a signed Gauss code.
This makes some arbitrary choice of orientation.
INPUT:
a signed Gauss code
OUTPUT:
a knot
EXAMPLES:
sage: W = Knots() sage: K1 = W.from_gauss_code([2, 1, 3, 2, 1, 3]) sage: K1.alexander_polynomial() t^1  1 + t
 from_table(n, k)#
Return a knot from its index in the Rolfsen table.
INPUT:
n
– the crossing numberk
– a positive integer
OUTPUT:
the knot \(K_{n,k}\) in the Rolfsen table
EXAMPLES:
sage: K1 = Knots().from_table(6,3); K1 Knot represented by 6 crossings sage: K1.alexander_polynomial() t^2  3*t^1 + 5  3*t + t^2 sage: K2 = Knots().from_table(8,4); K2 Knot represented by 9 crossings sage: K2.determinant() 19 sage: K2.signature() 2 sage: K3 = Knots().from_table(10,56); K3 Knot represented by 11 crossings sage: K3.jones_polynomial() t^10  3*t^9 + 6*t^8  9*t^7 + 10*t^6  11*t^5 + 10*t^4  7*t^3 + 5*t^2  2*t + 1 sage: K4 = Knots().from_table(10,100) sage: K4.genus() 4
REFERENCES:
 one()#
Return the unit of the monoid.
This is the trivial knot.
EXAMPLES:
sage: Knots().one() Knot represented by 0 crossings