Trace-range conjecture for unconditional twisted group-algebra completions

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Let Γ\Gamma be a torsion-free group such that (Γ,σ)(\Gamma,\sigma) satisfies the twisted Bost conjecture, and suppose that the classifying space BΓB\Gamma is a smooth, compact, oriented manifold. For an unconditional completion A(Γ,σ)\mathcal A(\Gamma,\sigma), write ω\omega for the cohomology class associated with the multiplier and let [tr][\operatorname{tr}] denote the induced trace map on K0(A(Γ,σ))K_0(\mathcal A(\Gamma,\sigma)). In even dimension 2n2n, choose generators a1(j),,ab2j(j)a_1(j),\ldots,a_{b_{2j}}(j) of H2j(BΓ,Z)H2j(BΓ,R)H^{2j}(B\Gamma,\mathbb Z)\cap H^{2j}(B\Gamma,\mathbb R), with b2j=dimH2j(BΓ,R)b_{2j}=\dim H^{2j}(B\Gamma,\mathbb R); in odd dimension 2n12n-1, choose generators a1(j),,ab2j1(j)a_1(j),\ldots,a_{b_{2j-1}}(j) of H2j1(BΓ,Z)H2j1(BΓ,R)H^{2j-1}(B\Gamma,\mathbb Z)\cap H^{2j-1}(B\Gamma,\mathbb R), with b2j1=dimH2j1(BΓ,R)b_{2j-1}=\dim H^{2j-1}(B\Gamma,\mathbb R). Trace-range conjecture. If dimBΓ=2n\dim B\Gamma=2n, then

[tr](K0(A(Γ,σ)))=Z+Zθ+j=1n1k=1b2jZrk,j,nθk(j),[\operatorname{tr}](K_0(\mathcal A(\Gamma,\sigma)))=\mathbb Z+\mathbb Z\theta+\sum_{j=1}^{n-1}\sum_{k=1}^{b_{2j}}\mathbb Z r_{k,j,n}\theta_k(j),

where θk(j)=[ωnjak(j)],[BΓ]\theta_k(j)=\langle[\omega^{n-j}\cup a_k(j)],[B\Gamma]\rangle, 2(2π)nθ=[ωn],[BΓ]2(2\pi)^n\theta=\langle[\omega^n],[B\Gamma]\rangle, and rk,j,nr_{k,j,n} are universal constants. If dimBΓ=2n1\dim B\Gamma=2n-1, then

[tr](K0(A(Γ,σ)))=Z+j=1n1k=1b2j1Zrk,j,nθk(j),[\operatorname{tr}](K_0(\mathcal A(\Gamma,\sigma)))=\mathbb Z+\sum_{j=1}^{n-1}\sum_{k=1}^{b_{2j-1}}\mathbb Z r'_{k,j,n}\theta_k(j),

where θk(j)=[ωnjak(j)],[BΓ]\theta_k(j)=\langle[\omega^{n-j}\cup a_k(j)],[B\Gamma]\rangle and rk,j,nr'_{k,j,n} are universal constants. The formulas extend the computed four-dimensional trace-range result to arbitrary even and odd dimensions, subject to the twisted Bost conjecture.

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Primary source

Varghese Mathai, “Heat kernels and the range of the trace on completions of twisted group algebras”, arXiv:math/0606790 (2006).

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