# A stratified homotopy hypothesis

### David Ayala

Montana State University, Bozeman, USA### John Francis

Northwestern University, Evanston, USA### Nick Rozenblyum

University of Chicago, USA

## Abstract

We show that conically smooth stratified spaces embed fully faithfully into $\infty$-categories. This articulates a stratified generalization of the homotopy hypothesis proposed by Grothendieck. Hence, each $\infty$-category defines a stack on conically smooth stratified spaces, and we identify the descent conditions it satisfies. These include $\mathbb R^1$-invariance and descent for open covers and blow-ups, analogous to sheaves for the h-topology in $\mathbb A^1$-homotopy theory. In this way, we identify $\infty$-categories as striation sheaves, which are those sheaves on conically smooth stratified spaces satisfying the indicated descent. We use this identification to construct by hand two remarkable examples of $\infty$-categories: $\mathcal B\mathsf{un}$, an $\infty$-category classifying constructible bundles; and $\mathcal E\mathsf {xit}$, the absolute exit-path $\infty$-category. These constructions are deeply premised on stratified geometry, the key geometric input being a characterization of conically smooth stratified maps between cones and the existence of pullbacks for constructible bundles.

## Cite this article

David Ayala, John Francis, Nick Rozenblyum, A stratified homotopy hypothesis. J. Eur. Math. Soc. 21 (2019), no. 4, pp. 1071–1178

DOI 10.4171/JEMS/856