Splitting of Tensor Products and Intermediate Factor Theorem: Continuous Version

TL;DR

This paper establishes conditions under which intermediate subalgebras in tensor product inclusions are themselves tensor products, extending classical splitting results and applying to topological group actions and boundary theory.

Contribution

It generalizes splitting theorems for $C^*$-algebras to a continuous setting and introduces the notion of uniformly rigid actions, providing new criteria for tensor product decompositions.

Findings

Every intermediate subalgebra under certain conditions is a tensor product.

Topological version of the Intermediate Factor theorem is proved.

Necessary conditions for the splitting results are identified.

Abstract

Let $G$ be a discrete group. Given unital $G$-$C^*$-algebras $\mathcal{A}$ and $\mathcal{B}$, we give an abstract condition under which every $G$-subalgebra $\mathcal{C}$ of the form $\mathcal{A}\subset \mathcal{C}\subset \mathcal{A}\otimes_{\text{min}}\mathcal{B}$ is a tensor product. This generalizes the well-known splitting results in the context of $C^*$-algebras by Zacharias and Zsido. As an application, we prove a topological version of the Intermediate Factor theorem. When a product group $G=\Gamma_1\times\Gamma_2$ acts (by a product action) on the product of corresponding $\Gamma_i$-boundaries $\partial\Gamma_i$, using the abstract condition, we show that every intermediate subalgebra $C(X)\subset\mathcal{C}\subset C(X)\otimes_{\text{min}}C(\partial\Gamma_1\times \partial\Gamma_2)$ is a tensor product (under some additional assumptions on $X$). This can be considered as a topological version of the Intermediate Factor theorem. We prove that our assumptions are necessary and cannot generally be relaxed. We also introduce the notion of a uniformly rigid action for $C^*$-algebras and use it to give various classes of inclusions $\mathcal{A}\subset \mathcal{A}\otimes_{\text{min}}\mathcal{B}$ for which every invariant intermediate algebra is a tensor product.