On Small Types in Univalent Foundations
Tom de Jong, Martín Hötzel Escardó
TL;DR
This work analyzes predicative, constructive univalent foundations without propositional resizing, focusing on how largeness phenomena constrain common complete-structure notions. It introduces $\delta_{\mathcal{V}}$-complete posets to capture directed- and bounded-complete structures and proves that nontrivial examples are necessarily large, while nontrivial locally small posets lack decidable equality unless resizing is admitted; positivity strengthens this to full excluded middle. The authors show that Tarski-style fixed-point results fail predicatively in nontrivial cases and examine the definability and interaction of propositional truncations, set quotients, and set replacement, establishing that small set quotients yield small suprema for ordinals. They also compare completeness with respect to small families vs all subsets and demonstrate how univalence and resizing interact to shape universe-lifting behavior, thereby clarifying predicative foundations for domain-theoretic constructions in univalent type theory. These results illuminate fundamental limits on predicative univalent mathematics and guide the design of size-conscious definitions of suprema, quotients, and truncations.
Abstract
We investigate predicative aspects of constructive univalent foundations. By predicative and constructive, we respectively mean that we do not assume Voevodsky's propositional resizing axioms or excluded middle. Our work complements existing work on predicative mathematics by exploring what cannot be done predicatively in univalent foundations. Our first main result is that nontrivial (directed or bounded) complete posets are necessarily large. That is, if such a nontrivial poset is small, then weak propositional resizing holds. It is possible to derive full propositional resizing if we strengthen nontriviality to positivity. The distinction between nontriviality and positivity is analogous to the distinction between nonemptiness and inhabitedness. Moreover, we prove that locally small, nontrivial (directed or bounded) complete posets necessarily lack decidable equality. We prove our results for a general class of posets, which includes e.g. directed complete posets, bounded complete posets, sup-lattices and frames. Secondly, the fact that these nontrivial posets are necessarily large has the important consequence that Tarski's theorem (and similar results) cannot be applied in nontrivial instances. Furthermore, we explain that generalizations of Tarski's theorem that allow for large structures are provably false by showing that the ordinal of ordinals in a univalent universe has small suprema in the presence of set quotients. The latter also leads us to investigate the inter-definability and interaction of type universes of propositional truncations and set quotients, as well as a set replacement principle. Thirdly, we clarify, in our predicative setting, the relation between the traditional definition of sup-lattice that requires suprema for all subsets and our definition that asks for suprema of all small families.