Comprehension and quotient structures in the language of 2-categories
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Lawvere observed in his celebrated work on hyperdoctrines that the set-theoretic schema of comprehension can be elegantly expressed in the functorial language of categorical logic, as a comprehension structure on the functor p:ℰ→ℬ defining the hyperdoctrine. In this paper, we formulate and study a strictly ordered hierarchy of three notions of comprehension structure on a given functor p:ℰ→ℬ, which we call (i) comprehension structure, (ii) comprehension structure with section, and (iii) comprehension structure with image. Our approach is 2-categorical and we thus formulate the three levels of comprehension structure on a general morphism p:𝐄→𝐁 in a 2-category 𝒦. This conceptual point of view on comprehension structures enables us to revisit the work by Fumex, Ghani and Johann on the duality between comprehension structures and quotient structures on a given functor p:ℰ→ℬ. In particular, we show how to lift the comprehension and quotient structures on a functor p:ℰ→ℬ to the categories of algebras or coalgebras associated to functors F_ℰ:ℰ→ℰ and F_ℬ:ℬ→ℬ of interest, in order to interpret reasoning by induction and coinduction in the traditional language of categorical logic, formulated in an appropriate 2-categorical way.
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