Extensivity of categories of relational structures

Abstract

We prove that the category of models of any relational Horn theory satisfying a mild syntactic condition is infinitely extensive. Central examples of such categories include the categories of preordered sets and partially ordered sets, and the categories of small $\mathcal{V}$-categories, (symmetric) pseudo-$\mathcal{V}$-metric spaces, and (symmetric) $\mathcal{V}$-metric spaces for a commutative unital quantale $\mathcal{V}$. We also explicitly characterize initial sources and final sinks in such categories, and in particular embeddings and quotients.

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