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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 V-categories, (symmetric) pseudo-V-metric spaces, and (symmetric) V-metric spaces for a commutative unital quantale V. We also explicitly characterize initial sources and final sinks in such categories, and in particular embeddings and quotients.
@article{TAC_2022_38_a22, author = {Jason Parker}, title = {Extensivity of categories of relational structures}, journal = {Theory and applications of categories}, pages = {898--912}, publisher = {mathdoc}, volume = {38}, year = {2022}, language = {en}, url = {http://geodesic.mathdoc.fr/item/TAC_2022_38_a22/} }
Jason Parker. Extensivity of categories of relational structures. Theory and applications of categories, Tome 38 (2022), pp. 898-912. http://geodesic.mathdoc.fr/item/TAC_2022_38_a22/