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A topos is a category that behaves very much like the category of sets and possesses a good notion of localization. Related to topos are: sheaves, presheaves, descent, stacks, localization,...
5
votes
Accepted
internal version of a flat functor?
A flat internal presheaf/discrete fibration $F \to C$ is simply one whose total category F is filtered in the internal sense (see Topos Theory 2.51 (filteredness), 4.31 (flatness); Elephant B.2.6.2, B …
3
votes
Lattice of subcategories: subobject classifier in Cat
Remember that $\mathbf{2}$ is (classically) the subobject classifier in Set, so that there is a natural bijection $\mathrm{Sub}(X) \cong \hom(X,\mathbf{2})$ given by pulling back along $\mathrm{true} …