A local-global principle for parametrized $\infty$-categories

TL;DR

This paper establishes a local-global principle for $$-categories over any base, showing they are determined by fibers, local data, and gluing data, with applications to mapping spaces and classification of categories over $$.

Contribution

It introduces a novel local-global framework for parametrized $$-categories, linking fibers, automorphisms, and presheaf data, and applies it to classify categories over a base.

Findings

Describes the $$-category of small $$-categories over $[1]$ using left fibrations.

Proves an end formula for mapping spaces of internal homs in $$-categories.

Shows $$-categories over any $$-category are classified by normal lax 2-functors.

Abstract

We prove a local-global principle for $\infty$-categories over any base $\infty$-category $\mathcal{C}$: we show that any $\infty$-category $\mathcal{B} \to \mathcal{C}$ over $\mathcal{C}$ is determined by the following data: the collection of fibers $\mathcal{B}_X$ for $X$ running through the set of equivalence classes of objects of $\mathcal{C}$ endowed with the action of the space of automorphisms $\mathrm{Aut}_X(\mathcal{B})$ on the fiber, the local data, together with a locally cartesian fibration $\mathcal{D} \to \mathcal{C}$ and $\mathrm{Aut}_X(\mathcal{B})$-linear equivalences $\mathcal{D}_X \simeq \mathcal{P}(\mathcal{B}_X)$ to the $\infty$-category of presheaves on $\mathcal{B}_X$, the gluing data. As applications we describe the $\infty$-category of small $\infty$-categories over $[1]$ in terms of the $\infty$-category of left fibrations and prove an end formula for mapping spaces of the internal hom of the $\infty$-category of small $\infty$-categories over $[1]$ and the conditionally existing internal hom of the $\infty$-category of small $\infty$-categories over any small $\infty$-category $\mathcal{C}.$ Considering functoriality in $\mathcal{C}$ we obtain as a corollary that the double $\infty$-category $\mathrm{CORR}$ of correspondences is the pullback of the double $\infty$-category $\mathrm{PR}^L$ of presentable $\infty$-categories along the functor $\infty\mathrm{Cat} \to \mathrm{Pr}^L$ taking presheaves. We deduce that $\infty$-categories over any $\infty$-category $\mathcal{C}$ are classified by normal lax 2-functors.