# Knots¶

AUTHORS:

• Miguel Angel Marco Buzunariz
class sage.knots.knot.Knot(data, check=True)

A knot.

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.

INPUT:

• data – see Link for the allowable inputs
• check – optional, default True. If True, 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


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

connected_sum(other)

Return the oriented connected sum of self and other.

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 and other.

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: K = trefoil.connected_sum(rev_trefoil); K
Knot represented by 6 crossings
sage: K.braid()
s0^3*s1^-1*s0^-3*s1


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.

Element

alias of Knot

an_element()

Return the trefoil knot.

EXAMPLES:

sage: Knots().an_element()
Knot represented by 3 crossings

one()

Return the unit of the monoid.

This is the trivial knot.

EXAMPLES:

sage: Knots().one()
Knot represented by 0 crossings