The Kadison–Kaplansky conjecture states that the reduced C*-algebra of a torsion-free discrete group has no idempotents other than 0 and 1. It holds for groups satisfying the Baum–Connes conjecture. If we restrict focus to group algebras, there are stronger conjectures attributed to Kaplansky on zero divisors and units. I will discuss these conjectures and my counterexample to the unit conjecture.
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