Finiteness and finite domination in stratified homotopy theory

TL;DR

This paper investigates conditions under which stratified homotopy categories are finite or compact, providing criteria based on the homotopy types of strata and links, and explores differences between smooth and topological stratifications.

Contribution

It establishes new criteria linking stratification properties to finiteness and compactness of associated infinity-categories, and distinguishes between smooth and topological stratifications.

Findings

Stratification conditions imply compactness and finiteness of exit path categories.

Finite homotopy types of strata lead to finite exit path categories in smooth cases.

Counterexamples show topological stratifications can be compact but not finite.

Abstract

In this paper, we study compactness and finiteness of an $\infty$-category $\mathcal{C}$ equipped with a conservative functor to a finite poset $P$. We provide sufficient conditions for $\mathcal{C}$ to be compact in terms of strata and homotopy links of $\mathcal{C}\rightarrow P$. Analogous conditions for $\mathcal{C}$ to be finite are also given. From these, we deduce that, if $X\rightarrow P$ is a conically stratified space with the property that the weak homotopy type of its strata, and of strata of its local links, are compact (respectively finite) $\infty$-groupoids, then $\text{Exit}_P(X)$ is compact (respectively finite). This gives a positive answer to a question of Porta and Teyssier. If $X\rightarrow P$ is equipped with a conically smooth structure (e.g. a Whitney stratification), we show that $\text{Exit}_P(X)$ is finite if and only the weak homotopy types of the strata of $X\rightarrow P$ are finite. The aforementioned characterization relies on the finiteness of $\text{Exit}_P(X)$, when $X\rightarrow P$ is compact and conically smooth. We conclude our paper by showing that the analogous statement does not hold in the topological category. More explicitly, we provide an example of a compact $C^0$-stratified space whose exit paths $\infty$-category is compact, but not finite. This stratified space was constructed by Quinn. We also observe that this provides a non-trivial example of a $C^0$-stratified space which does not admit any conically smooth structure.