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Equivariant elliptic-complex index character and fixed-component formula

Statement

Assume full AC. Let T=(R/Z)r, r≥0, act smoothly by isometries on a compact smooth manifold M without boundary. Let E0,…,EN be smooth finite-rank Hermitian complex bundles and dj:C∞(M,Ej)→C∞(M,Ej+1) first-order differential maps with dj+1dj=0, setting d−1=dN=0. Their positive-frequency symbols use ∂xα↦(iξ)α. Assume the symbol sequence is exact for every nonzero real cotangent vector. A jointly smooth unitary bundle action has maps ρtj(x):Exj→Et⋅xj, with ρej(x)=I and ρstj(x)=ρsj(t⋅x)ρtj(x); set (Utjs)(x)=ρtj(t−1⋅x)s(t−1⋅x) and require Utj+1dj=djUtj.

Write d=⨁jdj, let d∗ be its formal adjoint for the invariant metrics and Riemannian volume, and put D=d+d∗. Its even-to-odd symbol defines the compact-support class σ∈KT,c0(T∗M). The cohomology groups Hj=ker⁡dj/im⁡dj−1 are finite dimensional. Define Ind⁡T(E∙)=∑j(−1)j[Hj]∈R(T), where R(T) is the Grothendieck ring of finite-dimensional continuous complex representations.

For any g∈T, write Mg=∐aFa for its finitely many connected fixed components, νa=(TFa)⊥⊂TM∣Fa, πa:T∗Fa→Fa, and ia:T∗Fa↪T∗M for extension of covectors by zero on νa. Put νa,C=νa⊗RC and ea=λ−1(νa,C∗)=∑k(−1)k[Λkνa,C∗]. Then

χInd⁡T(E∙)(g)=∑a∫T∗Fach⁡g,c(ia∗σ)πa∗ch⁡g(ea) πa∗Td⁡(TFa⊗RC).

For a symbol on the g-fixed base, ch⁡g,c means the compact-support relative Chern character weighted by the eigenvalue of g on each eigenbundle; for an absolute virtual bundle V, ch⁡g(V)=∑λλch⁡(Vλ). The Todd class is ∏jxj/(1−e−xj) in formal Chern roots, truncated in each degree. The integral is the oriented compact-support cohomological pairing on the cotangent bundle, oriented by dθ=∑jdξj∧dxj. The denominator has nonzero degree-zero term det⁡R(I−dg∣νa), so its positive-degree part is inverted by a finite nilpotent series. Its full characteristic class is retained for a positive-dimensional component. The degree-zero value of ch⁡g is the trace at g; it is distinct from the degree-zero rank of a Borel equivariant Chern character.

At an element where the fixed locus changes, the formula uses that element's actual fixed components and normal bundles. It applies also at g=e, when F=M and the normal denominator is 1; it asserts no transversally elliptic distribution formula. If the fixed components are isolated points, it specializes to

χInd⁡T(E∙)(g)=∑x∈Mg∑j(−1)jtr⁡(ρgj(x)∣Exj)∣det⁡R(I−dgx)∣.

This isolated-point assertion also holds assuming only countable choice, provided every fixed point is simple and the nonnegative selfadjoint Hodge realization of Δ=D2 has discrete spectrum, smooth eigensections, trace-class heat operators with smooth kernels for positive time, finite-dimensional cohomology, and zero eigenspace canonically isomorphic to that cohomology. Thus the earlier spectral isolated-point case retains its weaker choice assumption.

Facts & Assumptions

Given: The equivariant first-order elliptic complex and invariant metrics in the Statement; full AC for the general fixed-component assertion, or the separately stated countable-choice Hodge heat hypotheses for its isolated-point branch.

[F1]

The index and its character are the alternating cohomology representation and trace. Equivariant elliptic-complex index and character

[F2]

The positive-frequency even-to-odd symbol gives a compact-support equivariant triple, and the doubled real normal uses the exterior-multiplication Koszul convention. Equivariant compact-support symbol classes and the normal Koszul class

[L1]

Compact-symbol analytic realization identifies the complex's index with aT,M(σ). Analytic Gysin compatibility holds for a real equivariant embedding. On a base fixed by g, evaluation is the eigenvalue-weighted ordinary topological pushforward. The full analytic calculus, controlled excision, positive-order associated product and orthogonal Bott normalization are proved in its prerequisite chain. Analytic symbol index, normal embedding compatibility and ordinary pushforward

[L2]

After localization at Sg={s∈R(T):s(g)≠0}, the fixed inclusion reconstructs every class by σ=∑aia!((πa∗ea)−1ia∗σ). It proves smoothness and finiteness of the actual fixed components, the normal Koszul self-intersection, and invertibility of the normal class. Fixed-inclusion localization for equivariant compact symbol classes

[L3]

The trace-weighted ordinary compact-symbol pushforward is the cotangent integral of its relative Chern character times Td⁡(TFC), with precisely the stated orientation. It respects localized quotients and retains the full normal characteristic denominator. Trace-weighted compact symbol character and the cotangent pushforward

[L4]

With countable choice and the stated Hodge heat data, simple fixed points have heat-supertrace limit ∑xStr⁡(ρg(x))/∣det⁡R(I−dgx)∣. Relative tangent-groupoid heat-trace limit at simple fixed points

[A1]

Full AC is inherited from the new analytic and topological prerequisites for the general formula. The Axiom of Choice

[A2]

Only countable choice is used through the original local heat supplier in the isolated-point spectral branch. The Axiom of Countable Choice (ACω)

Proof

technique · Construct the general analytic-symbol reduction and fixed-inclusion reconstruction before evaluating the relative characteristic pushforward; retain the independent isolated heat argument

Given: The hypotheses and exact conventions in the Statement.

1.1givenF1F2L1A1algebra

Put b=σ1(d) at a nonzero real covector. The complex identity gives b2=0. The positive-frequency symbol of the formal adjoint is b∗ by integration by parts, so the symbol of D is b+b∗. Its square is bb∗+b∗b; a vector in its kernel is killed by both b and b∗, hence belongs to ker⁡b=im⁡b and is orthogonal to that image, so is zero. Thus its even-to-odd block is invertible off the zero section and represents σ in [F2]. By [L1] the cohomology is finite dimensional and its alternating representation is exactly aT,M(σ). This uses the general matrix-symbol analytic chain and does not assume a Clifford-module reduction.

2.1L1L2step 1.1algebra

Apply [L2] to this actual g. Let ba=(πa∗ea)−1ia∗σ in the localized compact-support group of T∗Fa. Its reconstruction is σ=∑aia!ba. The analytic index in [L1] is R(T)-linear and its Gysin compatibility applies to the real embedding Fa↪M, with exactly the doubled-real Koszul normal used by [L2]. Applying that homomorphism, and then evaluating the localized representations at g, gives χInd⁡T(E∙)(g)=∑aχaT,Fa(ba)(g). The sum is finite; all denominators lie in Sg, so evaluation is defined.

3.1L1L3step 2.1algebra

The closure of the cyclic group generated by g fixes Fa pointwise. The weightwise assertion in [L1] expresses each character on the right of step 2.1 as the weighted ordinary topological pushforward of ba. Apply [L3]. The relative character is multiplicative by an absolute bundle class, so ch⁡g,c(ba)=ch⁡g,c(ia∗σ)/(πa∗ch⁡g(ea)). Its compact support is inherited from the restricted symbol; the other factors are pulled back from compact Fa. The resulting cotangent integral is precisely the displayed term. Sum over a to prove the general formula.

4.1F2L2L3step 3.1algebra

The normal has no g-fixed vector by [L2], so the constant term of ch⁡g(ea) is the nonzero determinant det⁡C(I−g∣νa,C∗)=det⁡R(I−dg∣νa). Orthogonality makes it positive: conjugate nonreal eigenvalues give positive paired factors, and the only possible real eigenvalue different from 1 is −1. Positive-degree cohomology of compact Fa is nilpotent by its finite dimension, so dividing by that constant and taking a finite geometric series defines the inverse. If Fa is a point its Todd factor is 1 and its symbol triple is the virtual graded fiber, so step 3.1 gives the isolated formula. Empty fixed loci give the empty sum, rank-zero normals give denominator 1, and the identity element uses F=M. At an element with larger fixed locus the same proof uses its larger tangent fixed subspace, so no zero normal eigenvalue is retained as a denominator.

5.1givenF1L4A2algebra∎

For the separate countable-choice branch, assume only its stated spectral data. The unitary action commutes with d and hence with d∗ and D. On each positive Δ-eigenspace with eigenvalue λ, the map λ−1/2D is an equivariant isomorphism between even and odd parity. Their traces therefore cancel. Trace-classness makes the heat trace series absolutely convergent, so these cancellations can be summed. The zero eigenspace is the cohomology by the branch's Hodge hypothesis, giving Str⁡(Uge−tΔ)=χInd⁡T(E∙)(g) for every t>0. The limit in [L4] proves the isolated-point formula under [A2], independently of [A1] and the general-branch steps. Thus the original weaker-choice spectral assertion is retained as well as the full fixed-component claim.

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