Separability in homotopical algebra

TL;DR

This paper explores the concept of separable algebras within symmetric monoidal stable ∞-categories, linking them to tensor-triangulated categories, Azumaya algebras, and Hochschild homology, offering new insights and proofs in homotopical algebra.

Contribution

It introduces and compares separable algebras in ∞-categories to tensor-triangulated categories, studies ind-separability, and relates separable algebras to Azumaya algebras and Hochschild homology.

Findings

Separable algebras are largely controlled by the homotopy category.

A new proof of the Goerss--Hopkins--Miller theorem is provided.

Centers of separable algebras are shown to be separable in some cases.

Abstract

We study the notion of \emph{separable algebras} in the context of symmetric monoidal stable $\infty$-categories. In the first part of this paper, we compare this context to that of tensor-triangulated categories and show that separable algebras and their modules in a symmetric monoidal stable $\infty$-category are, in large parts, controlled by the (tensor-triangulated) homotopy category. We also study a variant of this notion, which we call ind-separability. Among other things, this provides a partially new proof of the Goerss--Hopkins--Miller theorem about the uniqueness of $\mathbb E_\infty$-structures on Morava $E$-theory. We later initiate a study of separable algebras \textit{\`a la} Auslander-Goldman by relating them to Azumaya algebras, and prove in some restrictive cases that centers of separable algebras are separable. Finally, we study the Hochschild homology of separable algebras and prove some descent results in topological Hochschild homology.