# A van Kampen theorem for toposes

### Marta Bunge and Stephen Lack

In this paper we introduce the notion of an extensive 2-category,
to be thought of as "2-category of generalized spaces". We consider
and extensive 2-category *K* equipped with a binary-product-preserving
pseudofunctor *C:K*^{op}-->**CAT**, which we think of as
specifying the "coverings" of the generalized spaces. We prove, in this
context, a van Kampen theorem which generalizes and refines one of Brown
and Janelidze. The local properties required in this theorem are stated
in terms of morphism of effective descent for the pseudofunctor *C*.
We specialize the general van Kampen theorem to the 2-category
**Top**_{S} of toposes bounded over an elementary
topos *S*, and to its full sub-2-category **LTop**_{S}
determined by the locally connected toposes, after showing both of these
2-categories to be extensive. We then consider three particular notions
of coverings on toposes corresponding respectively to local homeomorphisms,
covering projections, and unramified morphisms; in each case we deduce a
suitable version of a van Kampen theorem in terms of coverings and, under
further hypotheses, also one in terms of fundamental groupoids. An application
is also given to knot groupoids and branched coverings. Along the way we
are led to investigate locally constant objects in a topos bounded over an
arbitrary base topos *S* and to establish some new facts about them.

Click here to download a gzipped postscript file of the complete paper. This is the final version, to be published in
*Advances in Mathematics*.

Steve Lack
Last modified: Fri Jan 24 12:33:59 EST 2003