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RT torus operators and the state-space dimension formula

Statement

Assume full AC and its inherited countable-choice consequences. For the weighted RT functor of The weighted Reshetikhin–Turaev TQFT and modular-functor factorization and any complex modular ribbon fusion category, simple labels i1,…,in and genus g≥0,

dim⁡V(Σg;i1,…,in)=∑x∈IS0x 2−2g−n∏r=1nSirx.

No complex conjugation is part of this general formula. The unmarked torus has dimension ∣I∣ and basis bi:1→i⊗i∗⊂H. Orient its meridian/longitude basis (m,l) by ω(m,l)=1 and choose reference and object Lagrangian α=λ=Rm. The weight-zero positive Hopf mapping cylinder and positive core-twist cylinder have respectively the geometric classes

s+=(01−10),t=(1101)

(columns are the images of m,l), and their operators are

Sji=sji/D,Tji=δjiθi.

For this positive Hopf/core-twist pair the exact relations are

S2=C,(ST)3=κI,Cji=δj,i∗.

The inverse Hopf class is s−=s+−1=[0−110] and its weight-zero operator is S−1. That alternative pair satisfies (S−1T)3=κC. These conventions retain the actual positive Hopf and core-twist matrices; replacing a generator by its inverse changes its geometric class. In a nonselfdual category C need not be I.

Facts & Assumptions

Given: The modular category, state-space caps, geometric orientation and weighted lifts fixed in the Statement.

[F1]

Modular ribbon fusion category and Hopf-link data defines s,D,θ; The weighted Reshetikhin–Turaev TQFT and modular-functor factorization defines p+=∑iθidi2 and κ=p+/D; The modular Hopf-link matrix squares to D² times duality proves symmetry and S2=C with the dual-label convention. The full pointed cocycle data and radical criterion are Pointed ribbon categories from abelian 3-cocycles and A pointed metric-group category is modular exactly when its radical is trivial.

[F2]

Modular Hom state spaces have exact sewing inverses and the dimension formula proves the entire displayed dimension identity directly for the actual Hom state spaces, including zero, one and two marked spheres and all multiplicities.

[F3]

The weighted Reshetikhin–Turaev TQFT and modular-functor factorization proves the full weighted functor, geometric sewing and duality. RT capped operators: geometric Gram factors and exact signature gluing proves the exact torus cap correction N∣j=dj.

[F4]

Wall nonadditivity, RT cap references, and the Walker weight sign proves weight action κn and composition weight n2+n1+μW(R1λ,λ,R2tλ). The four-Lagrangian Maslov cocycle identity defines its sum-zero Walker form ψ(a,a′)=ω(a1,a2′) and repeated-argument vanishing.

[A1]

Full AC and countable choice are inherited exactly from [F3], with The Axiom of Choice and The Axiom of Countable Choice (ACω). The matrix and Maslov calculations below are finite.

Proof

1.1F1F2F3givenalgebra

Apply [F2] to the actual state space from [F3]. Its separating inverse resolves every simple summand and multiplicity, and its nonseparating inverse resolves H; hence it identifies the geometrically factored space with exactly the Hom space used in [F2]'s dimension calculation. The displayed formula holds for all g,n, including the exceptional spheres by [F2]. At g=1,n=0 every summand is 1, giving ∣I∣. The summand Hom⁡(1,i⊗i∗) has basis bi by rigidity and simplicity, so these are a basis of the unmarked torus space.

2.1F1F3F4step 1.1construct

Fix the standard oriented solid torus D2×S1, with m the positive disk-boundary and l the core direction. Its boundary orientation is (m,l). The reference opposite-cap reflection acts on this basis by r=diag⁡(1,−1). The positive Hopf gluing exchanges the meridian of one cap with the longitude of the other; its two disk-core orientations give the exchange matrix [0110]. Removing the fixed opposite reflection gives the stated s+, since rs+ is that exchange matrix. This fixes which Hopf generator is meant and its linking orientation. In particular m↦−l, l↦m, rather than the inverse convention. In the capped matrix coefficient use the composition-dual test yj=rj/dj because rjbj=dj. The positive Hopf graph with these two coupons evaluates to sji/dj. Its closed empty-surgery normalization is D−1 and its outgoing torus correction from [F3] is dj, giving exactly Sji=sji/D. The positive core twist is the actual solid-torus map (z,eiϕ)↦(eiϕz,eiϕ), so m↦m, l↦l+m and its core normal disk makes one positive full rotation. Its ribbon operator is therefore θi on the i summand with no off-diagonal terms. The cylinder Gram inverse cancels as in [F3], leaving exactly T. When λ=α, both terms of the correction a(R;λ,α) vanish by repeated arguments in [F4], so these really are the weight-zero matrices, not unspecified scalar multiples.

3.1F1F3F4step 2.1algebra

Put A=s+t=[01−1−1]. Direct multiplication gives A3=I and s+2=−I. Composing the weight-zero s+,t lifts introduces no weight because t−1α=α, so its Maslov triple has a repeated argument. Thus ST is the weight-zero lift of A. For its square the middle triple is (Aα,α,A−1α)=(Rl,Rm,R(l−m)). A sum-zero vector is (−cl,cm,c(l−m)), and its Walker form is ω(−cl,c′m)=cc′, so this Maslov index is +1. For its third power the first and third lines coincide: A2α=A−1α. The next correction is zero. Therefore the cubed weighted lift is (I,1) and [F4]'s positive weight action gives (ST)3=κI. Similarly the correction in s+s+ is μ(l,m,l)=0, so its operator is the weight-zero −I lift, equal to S2=C by [F1]. These are geometric weighted relations, not an appeal to the desired modular matrix identity.

4.1F1F3F4A1step 2.1step 3.1algebra∎

The weight-zero inverse lift of s+ is s−=s+−1: its inverse-composition correction has repeated first and third lines, so is zero. Its operator is consequently S−1 by functoriality. Set B=s−t=−A. It has exactly the same action as A on Lagrangian lines, hence the same accumulated cube weight +1, while B3=−I. Its cubed operator is therefore κZ(−I,0)=κC, proving the asserted inverse-generator alternative. All matrix entries, dualities and Maslov signs use the fixed orientation from step 2.1; an orientation or inverse change cannot be hidden in a projective-scalar phrase. For example take the actual pointed data G=Z/3, ω=1, r(a,b)=ζab, q(a)=ζa2 and pivotal identity. Bilinearity verifies both hexagons, and q(a+b)=q(a)q(b)ζ2ab verifies balancing; q(−a)=q(a) and the duality snakes hold with unit evaluation/coevaluation. The induced pivotal identity gives all dimensions1, since the Drinfeld scalar ζ−a2 times the twist is1. Its bicharacter ζ2ab has zero radical, so [F1] makes this an actual nonselfdual modular category. It has Sab=ζ2ab/3, Taa=ζa2 and κ=(1+2ζ)/3. Finite multiplication in Q[ζ,3] gives (ST)3=κI and (S−1T)3=κC, where C exchanges labels1 and2 and is not I. This corroborates the convention-sensitive general proof; the exact arithmetic diagnostic is recorded in the owner packet. The proof uses no positivity or selfduality assumption.

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