Research Information System
Theory and Applications of Categories, vol.41, no.10, pp.288-413, 2024 (SCI-Expanded)
In this article we develop formal category theory within augmented virtual double categories. Notably we formalise the classical notions of Kan extension, Yoneda embedding yA: A → Â, exact square, total category and ‘small’ cocompletion; the latter in an appropriate sense. Throughout we compare our formalisations to their corresponding 2-categorical counterparts. Our approach has several advantages. For instance, the structure of augmented virtual double categories naturally allows us to isolate conditions that ensure small cocompleteness of formal presheaf objectsÂ. Given a monoidal augmented virtual double category K with a Yoneda embedding yI: I → Î for its monoidal unit I we prove that, for any ‘unital’ object A in K that has a ‘horizontal dual’ A°, the Yoneda embedding yA: A → Â exists if and only if the ‘inner hom’ [A°, Î] exists. This result is a special case of a more general result that, given a functor F: K → L of augmented virtual double categories, allows a Yoneda embedding in L to be “lifted”, along a pair of ‘universal morphisms’ in L, to a Yoneda embedding in K.