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Operator algebras

Counterexamples to Baum–Connes and Kadison–Kaplansky

OpenAI research collection · 3 manuscripts · Source checked 2026-10-19

OpenAI's result summary

Disproves the coefficient-free reduced Baum–Connes conjecture through a failure of rational injectivity and a separate failure of surjectivity of assembly. A finitely generated torsion-free example witnesses the injectivity failure. Separately, a torsion-free group has a nontrivial projection in its reduced group C^(∗)-algebra, disproving the Kadison–Kaplansky conjecture.

Read the manuscripts

Short excerpts below come from OpenAI's abstracts. The full abstract and original PDF preserve the paper's precise scope.

MANUSCRIPT 01

A torsion-free counterexample to reduced Baum–Connes injectivity

We disprove rational injectivity of the coefficient-free reduced Baum–Connes assembly map for torsion-free groups. Specifically, we construct a finitely generated torsion-free discrete group whose degree-zero assembly map has a kernel class of infinite order.

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MANUSCRIPT 02

A torsion-free counterexample to the Kadison–Kaplansky projection conjecture

We construct a finitely generated torsion-free discrete group whose reduced group C^(∗)-algebra contains a projection other than zero and the identity. This disproves the Kadison–Kaplansky projection conjecture.

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MANUSCRIPT 03

An irrational-trace counterexample to reduced Baum–Connes

We construct a finitely generated discrete group whose reduced group C^(∗)-algebra contains a projection of irrational canonical trace. Its K_(0)-class lies outside the image of the coefficient-free reduced Baum–Connes assembly map. This disproves the coefficient-free reduced Baum–Connes conjecture for countable discrete groups.

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