Definable and Non-definable Notions of Structure

TL;DR

This paper explores the concept of definability of structures within fibrations, generalizing from properties to structures, providing criteria for when such structures are definable, with applications to algebraic weak factorisation systems.

Contribution

It develops a general theory of definability for structures on objects, recovering known notions and providing new criteria for definability in algebraic weak factorisation systems.

Findings

A sufficient criterion for cofibrantly generated awfs's to be definable.

Examples of definability in cubical sets and related constructions.

Identification of logical principles that prevent definability in certain models.

Abstract

Definability is a key notion in the theory of Grothendieck fibrations that characterises when an external property of objects can be accessed from within the internal logic of the base of a fibration. In this paper we consider a generalisation of definability from properties of objects to structures on objects, introduced by Shulman under the name local representability. We first develop some general theory and show how to recover existing notions due to B\'{e}nabou and Johnstone as special cases. We give several examples of definable and non definable notions o structure, focusing on algebraic weak factorisation systems, which can be naturally viewed as notions of structure on codomain fibrations. Regarding definability, we give a sufficient criterion for cofibrantly generated awfs's to be definable, generalising a construction of the universe for cubical sets, but also including some very different looking examples that do not satisfy tininess in the internal sense, that exponential functors have a right adjoint. Our examples of non definability include the identification of logical principles holding for the interval objects in simplicial sets and Bezem-Coquand-Huber cubical sets that suffice to show a certain definition of Kan fibration is not definable.