The naive 2-category$Cat(S)$ of internal categories in a category $S$ does not have enough equivalences in general, due to the failure of the axiom of choice in $S$. The functors which should be equivalences are called weak equivalences, and one often works with the localisation of $Cat(S)$ at the weak equivalences. Roughly speaking, a weak equivalence is a functor which is ‘fully faithful’ and ‘essentially surjective’, but these terms need to be interpreted appropriately.

The concept of weak equivalences first arose in work of Bunge and Paré on stack completions of internal categories.

Definition

Let $f:X\to Y$ be a functor between categories internal to some category $S$. $f$ is fully faithful if the following diagram is a pullback

To discuss the analogue of essential surjectivity, we need a notion of ‘surjectivity’, as this does not generalise cleanly from $Set$. If we are working in a topos, a natural choice is to take epimorphisms, but weaker ambient categories are sometimes needed. A natural choice is to work in a unary site, where the covers are taken as the ‘surjective’ maps.

Given a functor $f:X\to Y$ internal to a unary site $(S,J)$, $f$ is essentially $J$-surjective if the map $t\circ pr_2:X_0 \times_{f_0,Y_0,s}Y_1 \to Y_0$ is a $J$-cover.

We then define an internal functor to be a $J$-equivalence if it is fully faithful and essentially $J$-surjective.

M. Bunge, R. Paré, Stacks and equivalence of indexed categories, Cahiers de Topologie et Géométrie Différentielle Catégoriques, 20 no. 4 (1979), p. 373-399 NUMDAM

T. Everaert, R. W. Kieboom and T. Van der Linden, Model structures for homotopy of internal categories, Theory and Applications of Categories 15 (2005), no. 3, 66-94. (journal)

David Roberts, Internal categories, anafunctors and localisation, Theory and Applications of Categories, 26 (2012) No. 29, pp 788-829. (journal)

Revised on March 3, 2015 01:09:11
by David Roberts
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