Searcharxiv⌕ Search
Home/Archive/arXiv

arXiv · math/9907121

The trace on the K-theory of group C*-algebras

Thomas Schick
View original source ↗Read source document ↗Export metadata ↓

Abstract

The canonical trace on the reduced C*-algebra of a discrete group gives rise to a homomorphism from the K-theory of this C^*-algebra to the real numbers. This paper addresses the range of this homomorphism. For torsion free groups, the Baum-Connes conjecture and Atiyah's L2-index theorem implies that the range consists of the integers. If the group is not torsion free, Baum and Connes conjecture that the trace takes values in the rational numbers. We give a direct and elementary proof that if G acts on a tree and admits a homomorphism \alpha to another group H whose restriction to every stabilizer group of a vertex is injective, then the range of the trace for G, tr_G(K(C_r^*G)) is contained in the range of the trace for H, tr_H(K(C_r^*H)). This follows from a general relative Fredholm module technique. Examples are in particular HNN-extensions of H where the stable letter acts by conjugation with an element of H, or amalgamated free products G=H*_U H of two copies of the same groups along a subgroup U.

Explore related subjects

math.KTmath.FAmath.OA

Keep this discovery

BibTeXRIS

Thomas Schick. 1999-07-20. The trace on the K-theory of group C*-algebras. https://doi.org/10.1215/s0012-7094-01-10711-4

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related papers

arXiv2026

The Oka principle for \'etale Chow groups

The celebrated theorems of Shilov, Arens--Royden, and Forster give direct descriptions of the first three integral cohomology groups of the Gelfand spectrum of a commutative complex Banach algebra. In his 1974 ICM address, Taylor asked whether the higher cohomology groups admit descriptions in terms of the underlying ring. We give a solution to this question in even degrees: The \'etale (aka Lichtenbaum) Chow group in every codimension is canonically isomorphic to the corresponding even integral cohomology group of the Gelfand spectrum.

math.KT↗
arXiv2026

General linear and Steinberg groups over the Leavitt algebra $L_{\mathbb F_2}(1,2)$

Let $R=L_{\F_2}(1,2)$. We prove that $\GL_r(R)$ is integrally acyclic for every $r\geq1$ and that the canonical map $\St_r(R)\to\GL_r(R)$ is an isomorphism for every $r\geq3$. The homology calculation combines simultaneous extensions of ordered frames with scalar actions of the multiplicative groups of finite fields on their stabilizers. The presentation associated with the same frame complex defines a surjective section of the Steinberg map. An explicit finite presentation of $R^\times$ then follows from the theorem of Krsti\'c and McCool. We formulate separate criteria for acyclicity and for the Steinberg comparison over other rings.

math.KT↗
arXiv2026

The K-theory of uniform Roe algebras for coarse structures generated by finite-rank free abelian subgroups

For a uniformly locally finite coarse space $X$, the uniform Roe algebra $C_u^*(X)$ is the operator norm closure of the controlled operators on $\ell^2(X)$. The $K$-theory of uniform Roe algebras is known in asymptotic dimension zero, but it is not fully understood in higher dimensions. We compute $K_0(C_u^*(G,\mathcal E))$ and $K_1(C_u^*(G,\mathcal E))$ for every countable discrete abelian group $G$ and every finite-rank free abelian subgroup $H\leq G$, where $\mathcal E$ is the coarse structure generated by $H$. We use the Proietti--Yamashita spectral sequence to express the $K$-theory in terms of $H_*(H;\ell^\infty(G,\mathbb Z))$, which we then compute.

math.KT↗