Every primitive ideal of a C*-algebra is closed and prime. A long-standing problem of Dixmier asked whether the converse holds, and this was settled by Weaver in 2003, when he gave the first example [1] of a prime C*-algebra that is not primitive. Between primitive ideals and closed prime ideals lies the class of factor… Read More: Primitive, factor and prime ideals »
Question: Is pure if (and only if) is non-amenable? Here, denotes the reduced group C*-algebra of a discrete group . A C*-algebra is said to be \emph{pure} if it is Jiang-Su stable at the level of Cuntz semigroups, in the sense that Equivalently, the Cuntz semigroup is almost unperforated and almost divisible (see… Read More: Pure group C*-algebras »
Introduction: The study of automatic continuity for homomorphisms (multiplicative, linear maps) between Banach algebras has a long history. In 1960, Bade and Curtis [1] proved the existence of discontinuous homomorphisms between commutative Banach algebras in ZFC, showing that automatic continuity results can only be achieved by imposing additional hypotheses on the algebras , or on… Read More: Automatic continuity of homomorphisms »
Question: Is the minimal tensor product of two C*-algebras pure whenever one of them is? Update (February 2026): A partial positive answer has been given by Seth and Vilalta [6]. In particular, is pure whenever is simple and pure and is an ASH algebra. Background: Following Winter [1], a C*-algebra is said to be pure… Read More: Tensor products with pure C*-algebras »
Given elements and in a ring, the element is called an (additive) commutator. Given a C*-algebra , we use to denote the additive subgroup (equivalently, the linear subspace) generated by the set of additive commutators in . Note that is not necessarily a closed subspace. Given additive subgroups and , it is customary to use… Read More: Commutators and square-zero elements in C*-algebras »
It is known that every nuclear C*-algebra is exact, and that exactness passes to sub-C*-algebras. It follows that every sub-C*-algebra of a nuclear C*-algebra is exact. For separable C*-algebras, the converse holds. In fact, Kirchberg’s -embedding theorem shows that a separable C*-algebra is exact if and only if it embeds into the Cuntz algebra (which… Read More: Nonseparable, exact C*-algebras »
Does there exist a purely infinite C*-algebra that admits a tracial weight taking a finite, nonzero value? Here a C*-algebra is purely infinite if every element is properly infinite ( is Cuntz subequivalent to in ). This notion was introduced and studied by Kirchberg-Rørdam in [1] and [2]. Further, a weight on a C*-algebra is… Read More: Traces on purely infinite C*-algebras »
In [1], Blackadar and Handelman made two conjectures: Conjecture 1: (Below Theorem I.2.4 in [1]) Let be a unital C*-algebra. Then the set of lower-semicontinuous dimension functions is dense in the set of dimension functions. Conjecture 2: (Below Theorem II.4.4 in [1]) Let be a unital C*-algebra. Then the compact, convex set is a Choquet… Read More: The Blackadar-Handelman conjectures »
Let be a unital, simple C*-algebra of stable rank one. Does there exist a commutative sub-C*-algebra such that for every lower-semicontinuous function there exists an open subset such that for ? Here, denotes the Choquet simplex of normalized -quasitraces on (if is exact, then this is just the Choquet simplex of tracial states on ),… Read More: Realizing Cuntz classes in commutative subalgebras »
Question: Is every separable C*-algebra an inductive limit of semiprojective C*-algebras? This question was first raised by Blackadar in [1]. If we think of C*-algebras as noncommutative topological spaces, then semiprojective C*-algebras are noncommutative absolute neighborhood retracts (ANRs). It is a classical result from shape theory that every metrizable space is homeomorphic to an inverse… Read More: Inductive limits of semiprojective C*-algebras »
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