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A braiding alone does not define a link trace

Statement refuted

The claim refuted is: a braiding on a monoidal category suffices to define a link trace, that is, to evaluate the closure of every braid. In Vectk with the transposition braiding the canonical braid actions exist by An object of a braided category carries canonical braid actions, but the categorical closure of even the one-braid e∈B1 has no value: closing a band requires evaluation and coevaluation maps, which exist only for dualizable objects, and k[x]∈Vectk has no dual (as proved below from the finite tensor sum and zig-zag identity). Even for rigid categories the braiding and chosen duals need not determine one ribbon evaluation: over a field of characteristic ≠2, finite-dimensional super vector spaces with their sign braiding admit both the identity twist and the parity twist. On the odd line these give respective unknot evaluations −1 and 1, as computed below. Thus neither existence in a non-rigid category nor uniqueness of a ribbon evaluation in a rigid one follows from a braiding alone.

Facts & Assumptions

Given: the symmetric monoidal category Vectk with the transposition braiding, its infinite-dimensional object k[x], and the one-braid e∈B1.

[F1]

A braiding alone yields canonical braid actions ρn ⁣:Bn→Aut⁡(X⊗n), with no further structure (An object of a braided category carries canonical braid actions).

[F2]

The trace formulas require evaluation and coevaluation maps, and a closure evaluation is typed on a morphism into a double dual; the pivotal comparison used for closures is jX=uXθX, where u is the Drinfeld morphism and θ the twist. The Drinfeld morphism need not be monoidal; its tensor obstruction is precisely the double braiding (The ribbon evaluation of an X-colored closed braid, A braided rigid category has a Drinfeld morphism).

[F4]

Ribbon twists obey balancing and dual-compatibility (Twist and ribbon structure), and the left trace is the evaluation--coevaluation composite of The categorical trace of a morphism into the double dual.

[F3]

A left dual has evaluation and coevaluation maps satisfying the zig-zag identities (Left dual and right dual object). Every element of an algebraic tensor product is a finite sum of elementary tensors (The tensor product M⊗RN from the additive group underlying the free Z-module on M×N, elementary tensors, and finite tensor sums).

Counterexample

1.1F1given

The braiding already gives the actions. In Vectk with the transposition braiding the canonical actions ρn of [F1] exist for every n, even though the category is not rigid. Hence whatever is missing from a link trace is not the braid action.

1.2F2F3F4given

The closure of the one-braid has no value in the non-rigid model. For the object X coloring the band, the left trace of jX:X→X∨∨ begins with coev⁡X, applies jX⊗1X∨, and ends with ev⁡X∨:X∨∨⊗X∨→1 [F2, F4]. If X=k[x] had a left dual Y, write coev⁡(1)=∑i=1Nvi⊗yi, a finite sum by [F3]. The zig-zag identity would give v=∑ivi ev⁡(yi⊗v) for every v∈X, putting all of X in the finite-dimensional span of the vi. The monomials 1,x,x2,… are linearly independent, so this is impossible. The right-dual version is the same mirror argument. Therefore the closure of e has no categorical trace value for X=k[x], and a braiding alone does not evaluate it.

1.3F2F3F4constructalgebra

A rigid model with two different ribbon evaluations. Take finite-dimensional Z/2-graded vector spaces over a field of characteristic ≠2, with even linear maps and c(v⊗w)=(−1)∣v∣∣w∣w⊗v. Graded duals, ordinary evaluation and basis coevaluation obey the zig-zags. The sign rule is natural, and both hexagons follow from (−1)p(q+r)=(−1)pq(−1)pr; the double braiding is the identity. Both θ0=1 and θ1(v)=(−1)∣v∣v are natural monoidal automorphisms preserving duals, hence ribbon twists by [F4]. On the odd line L=ke with odd dual basis e∨, the Drinfeld composite of [F2] sends e to −e∨∨: its sole crossing is cL,L∨(e⊗e∨)=−e∨⊗e. Thus jL0=−1 and jL1=1 under the usual double-dual identification; coevaluation 1↦e⊗e∨ and evaluation e∨∨⊗e∨↦1 give left traces −1 and 1. The same braiding and duals therefore give different ribbon unknot values.

2.1step 1.1step 1.2step 1.3∎

Conclusion. The non-rigid witness lacks the dual pair needed for closure, and the rigid witness has two different ribbon evaluations for the same braiding and chosen duals. Hence it does not suffice to define a link trace, and the claim is refuted.

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