Profunctorial algebras
Abstract
We provide a bicategorical generalization of Barr's landmark 1970 paper, in which he describes how to extend Set-monads to relations and uses this to characterize topological spaces as the relational algebras of the ultrafilter monad. With two-sided discrete fibrations playing the role of relations in a bicategory, we first describe how to extend pseudomonads on a bicategory to skew monads on its bicategory of two-sided discrete fibrations, and we characterize in terms of exact squares when these extensions are themselves pseudomonads. As a wide class of examples, we show that every Set-monad induces a pseudomonad on the 2-category of categories admitting a skew extension to profunctors, and in a few relevant cases we introduce suitable quotients also extending to profunctors. Among the latter, we then focus on the ultracompletion pseudomonad, whose pseudoalgebras are ultracategories: we characterize the normalized lax algebras of its profunctorial extension as ultraconvergence spaces, a recently-introduced categorification of topological spaces.
Cite
@article{arxiv.2601.22721,
title = {Profunctorial algebras},
author = {Quentin Aristote and Umberto Tarantino},
journal= {arXiv preprint arXiv:2601.22721},
year = {2026}
}
Comments
Generalized the extension theorem to skew monads and added explicit string diagrammatic proofs thereof; fixed an erroneous claim on the ultracompletion pseudomonad preserving exact squares; introduced left skew monads allowing for lax algebras