Projective covers, doctrines of algebras and the relational quotient completion

Abstract

The extensional quotient completion of relational doctrines provides a common generalization of both the exact completion of categories with weak finite limits and the elementary quotient completion of existential elementary doctrines. In this paper, we study projective objects in relational doctrines with quotients, characterizing those obtained through the extensional quotient completion as those admitting a projective cover. We apply this result to doctrines of algebras for monads on relational doctrines with quotients, describing in which cases these arise as the extensional quotient completion of their restriction to (appropriate subcategories of) free algebras. This extends a similar result for monadic categories over exact ones, covering also more examples such as monads over the category of metric spaces giving rise to variants of quantitative algebras.