# Categorical structures for type theory in univalent foundations

## TL;DR

This paper compares algebraic structures used for modeling dependent type theories within univalent foundations, establishing equivalences and transfer properties, and formalizing results in Coq.

## Contribution

It introduces a comparative analysis of categorical structures in univalent type theory, including new transfer results and the concept of relative universes.

## Key findings

- Under univalence, some structures are equivalent.
- Structures can be transferred along equivalences of categories.
- Results are formalized in Coq within the UniMath library.

## Abstract

In this paper, we analyze and compare three of the many algebraic structures that have been used for modeling dependent type theories: categories with families, split type-categories, and representable maps of presheaves. We study these in univalent type theory, where the comparisons between them can be given more elementarily than in set-theoretic foundations. Specifically, we construct maps between the various types of structures, and show that assuming the Univalence axiom, some of the comparisons are equivalences. We then analyze how these structures transfer along (weak and strong) equivalences of categories, and, in particular, show how they descend from a category (not assumed univalent/saturated) to its Rezk completion. To this end, we introduce relative universes, generalizing the preceding notions, and study the transfer of such relative universes along suitable structure. We work throughout in (intensional) dependent type theory; some results, but not all, assume the univalence axiom. All the material of this paper has been formalized in Coq, over the UniMath library.