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The HOMFLYPT polynomial from the Hecke Markov trace

Definition

Assume AC for the arbitrary-braid closure convention. Let R be the coefficient ring of The HOMFLYPT coefficient ring with its elements v,z,s,u,l,m and the relations s2=v, u2=z−/z, l=us, m=s−s−1, and let β∈Bn with closure β^ (The closure of a geometric braid). Write e(β) for the exponent sum of The exponent sum of a braid, πn:Bn→H(n)× for the homomorphism of The Hecke generators satisfy the Artin relations and are units, and tr⁡n for the Ocneanu trace of The Ocneanu Markov trace exists and is unique. The HOMFLYPT polynomial from the Hecke Markov trace is P(β^):=ue(β) α n−1 tr⁡n(πn(β))∈R,α:=(uz)−1, for n≥1. For the empty link, the closure of the unique braid in B0, set P(∅):=α−1=uz separately; no tr⁡0 is used. In the localization R[m−1], the identity l−1−l=mα from The HOMFLYPT coefficient ring gives α=l−1−lm,u=l s−1. Thus the image of P(β^) in R[m−1] is ue(β)(l−1−lm)n−1tr⁡n(πn(β)).

Well-formedness. e(β) and πn(β) depend only on the braid element, not on the chosen Artin word, and tr⁡n(πn(β)) lies in Λ and is mapped to R by the coefficient-ring structure map; u and α are elements of R, with u a unit, so the displayed product is a well-defined element of R.

Caveats. The construction as displayed is a function on braids on a fixed number of strands; it depends on the braid representative a priori, and the statement that it is independent of the representative of an oriented link is the content of The Hecke trace construction is an oriented link invariant ↗, not of this definition. The coefficient ring R is the formal localised ring of The HOMFLYPT coefficient ring; the normalisation α=(uz)−1 is the one that makes the two Markov stabilisations scale by the same factor, as proved in clauses (2)-(3) of that invariance theorem.

Facts & Assumptions

Given: AC (The Axiom of Choice), the coefficient ring R of The HOMFLYPT coefficient ring, an integer n≥1, a braid β∈Bn and its closure β^. AC implies countable choice (AC implies DC implies countable choice), the hypothesis used by the arbitrary-braid closure convention in The closure of a geometric braid; the trace formula itself is algebraic.

[F1]

R is a commutative Λ=Z[v±1,z]-algebra; v,z,u,s,l are units, s2=v, u2=z−/z with z−=v−1(z+1−v), m=s−s−1, and l−1−l=m(uz)−1 (The HOMFLYPT coefficient ring). In R[m−1] one may divide this identity by m.

[F2]

The exponent sum e:Bn→Z is the unique homomorphism with e(σi)=1, and for every Artin word β=σi1ε1⋯σikεk one has e(β)=∑rεr independently of the word (The exponent sum of a braid).

[F3]

The assignment σi↦Ti induces a group homomorphism πn:Bn→H(n)× with πn(σi1ε1⋯σikεk)=Ti1ε1⋯Tikεk for every Artin word (The Hecke generators satisfy the Artin relations and are units).

[F4]

The Ocneanu trace is a family of Λ-linear maps tr⁡n:H(n)→Λ satisfying (M1)--(M4), and it satisfies tr⁡n+1(xTny)=ztr⁡n(xy) for all x,y∈H(n) (The Ocneanu Markov trace exists and is unique); in particular tr⁡n(πn(β))∈Λ, with its image used in R.

[F5]

The closure β^ of a braid β∈Bn is an oriented link in S3 (The closure of a geometric braid).

Proof

1.1F1F2F3F4

Well-formedness of the factors. By [F2] the integer e(β) depends only on the element β; by [F3] the element πn(β)∈H(n)× depends only on β and not on the Artin word; by [F4] the trace of that element lies in Λ and has a specified image in R. The elements u and α=(uz)−1 of R exist because u and z are units of R by [F1]. Hence the product ue(β)αn−1tr⁡n(πn(β)) is a well-defined element of R, and it is computed from β alone, not from a word.

1.2F1algebra

The identities in the (l,m) variables. By [F1] one has l−1−l=m(uz)−1=mα. In the localization R[m−1] this gives α=(l−1−l)/m. Also l=us and s is a unit, so u=ls−1. Substituting these into the definition gives the displayed formula for the image of P in that localization.

2.1F1F2F3F5step 1.1∎

Dependence on the representative. The closure β^ is defined for every braid β∈Bn by [F5]; the definition produces an element P(β^)∈R for each braid, and no claim that two braids with isotopic closures give the same value is made here: that is exactly the statement proved in The Hecke trace construction is an oriented link invariant ↗. At n=0, the separate value α−1 exists because u,z are units; no stabilization starts at B0.

Remarks

  • The coefficient form in R[m−1] is used by The HOMFLYPT skein relation; the skein identity itself holds already in R.

  • The normalisation α=(uz)−1 is forced by the two Markov moves: the positive stabilisation multiplies the trace by z and the negative one by z−, and the two relations uαz=1 and u2=z−/z of The HOMFLYPT coefficient ring are precisely what make the two normalising factors uαz and u−1αz− equal to 1; see The Hecke trace construction is an oriented link invariant ↗.

  • The unknot is the closure of 1∈B1 and has P=1 because tr⁡1(1)=1 by (M1); the empty product n−1=0 contributes α0=1.

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