Operator Algebras of Universal Quantum Homomorphisms

TL;DR

This paper investigates the construction and properties of universal quantum operator algebras generated by unital *-homomorphisms between C*-algebras, covering finite and infinite dimensional cases, and explores their quantum semigroup structures.

Contribution

It introduces the concept of universal unital C*-algebras of quantum homomorphisms, analyzes their properties in finite and infinite dimensional cases, and studies associated quantum semigroup structures.

Findings

Universal algebra exists for finite-dimensional B

Properties like RFD, primitiveness, UCT are studied

Infinite-dimensional case leads to a locally C*-algebra

Abstract

Given two unital C*-algebras $A$ and $B$, we study, when it exists, the universal unital $C^*$-algebra $\mathcal{U}(A,B)$ generated by the coefficients of a unital $*$-homomorphism $\rho\,:\, A\rightarrow B\otimes\mathcal{U}(A,B)$. When $B$ is finite dimensional, it is well known that $\mathcal{U}(A,B)$ exists and we study in this case properties LP, RFD, primitiveness and the UCT as well as $K$-theory. We also construct a reduced version of $\mathcal{U}(A,B)$ for which we study exactness, nuclearity, simplicity, absence of non-trivial projection and $K$-theory. Then, we consider the von Neumann algebra generated by the reduced version and study factoriality, amenability, fullness, primeness, absence of Cartan, Connes' invariants, Haagerup property and Connes' embeddability. Next, we consider the case when $B$ is infinite dimensional: we show that for any non-trivial separable unital $C^*$-algebra $A$, $\mathcal{U}(A,B)$ exists if and only if $B$ is finite dimensional. Nevertheless, we show that there exists a unique unital locally $C^*$-algebra generated by the coefficients of a unital continuous $*$-homomorphism $\rho\,:\, A\rightarrow B\otimes\mathcal{U}(A,B)$. Finally, we study a natural quantum semigroup structure on $\mathcal{U}(A,A)$.