Accessible categories, set theory, and model theory: an invitation

TL;DR

This paper introduces accessible categories and explores their applications in set theory, model theory, homotopy theory, and algebra, highlighting recent developments and providing new proofs of classical equivalences.

Contribution

It offers a comprehensive, self-contained overview of accessible categories, including recent advances, connections to other fields, and a novel proof related to saturated and homogeneous models.

Findings

Recent progress on presentability ranks and stable independence.

Connections between accessible categories and homotopy theory.

A new proof of the equivalence between saturated and homogeneous models.

Abstract

We give a self-contained introduction to accessible categories and how they shed light on both model- and set-theoretic questions. We survey for example recent developments on the study of presentability ranks, a notion of cardinality localized to a given category, as well as stable independence, a generalization of pushouts and model-theoretic forking that may interest mathematicians at large. We give many examples, including recently discovered connections with homotopy theory and homological algebra. We also discuss concrete versions of accessible categories (such as abstract elementary classes), and how they allow nontrivial `element by element' constructions. We conclude with a new proof of the equivalence between saturated and homogeneous which does not use the coherence axiom of abstract elementary classes.