Homotopy Posets, Postnikov Towers, and Hypercompletions of $\infty$-Categories

TL;DR

This paper extends homotopical concepts to $( , )$-categories, introduces homotopy posets indexed by categorical disks, and develops a categorical Postnikov tower framework.

Contribution

It generalizes homotopical notions to higher categories, constructs homotopy posets, and establishes a Postnikov tower approach for $( , )$-categories.

Findings

Homotopy posets are indexed by boundaries of categorical disks.

A categorical Postnikov tower converges for $( , )$-categories.

The subcategory of Postnikov complete $( , )$-categories is characterized by inverting coinductive equivalences.

Abstract

We show that basic homotopical notions such as homotopy sets and groups, connected and truncated maps, cellular constructions and skeleta, etc., extend to the setting of $(\infty,\infty)$-categories, as well as to presentable categories enriched in $(\infty,\infty)$-categories under the Gray tensor product. The homotopy posets of an $(\infty,\infty)$-category are indexed by boundaries of categorical disks; in particular, there is a fundamental poset for each pair of objects, which we regard as a oriented point where the source and target objects have opposite orientation. In contrast to the situation in topology, weakly contractible geometric building blocks such as oriented polytopes typically have nontrivial homotopy posets. The homotopy posets assemble to form an oriented analogue of the long exact sequence of a fibration and form the layers of a categorical Postnikov tower, which converges for any $(\infty,n)$-category but not for general $(\infty,\infty)$-categories. We show that the full subcategory consisting of the Postnikov complete $(\infty,\infty)$-categories is obtained by inverting the coinductive equivalences and canonically identifies with the limit of the categories of $(\infty,n)$-categories taken along the truncation functors. We also study truncated morphisms in general oriented categories and connected morphisms in presentable oriented categories.