## RESUMO / ABSTRACT

23 Janeiro 1997, 14:30

**G. M. Kelly**, University of Sydney

*On the reflectiveness of coverings in topology, geometry and algebra*

Janelidze has defined a very general notion of Galois theory, where the basic
data consist of a category C, a reflective full subcategory X, and a class S
of the maps in C, with these data subject to mild assumptions. We can define
the TRIVIAL COVERINGS f: A --> B in C to be those maps in S which are the
pullbacks of their reflexions into X; then a map f: A --> B in C is a COVERING
if it lies in S and is LOCALLY a trivial covering, in the sense that its
pullback along some effective descent map p: E --> B is a trivial covering.
This notion of covering includes many classical concepts. When C is the
topos ofsheaves on a connected and locally-connected topological space and K
is the
category Set of constant sheaves, the coverings are the covering spaces in the
classical geometrical sense; when C is a more general connected and locally-
-connected topos, we get the covering theory of Barr and Diaconescu; when C is
the dual of the category of commutative rings, the coverings are the quasi-
-separable ring extensions; when C and X are suitable varieties of universal
algebras, the coverings are called CENTRAL EXTENSIONS, generalizing the
usual such concepts for groups and algebras; and so on.
The central point of Janelidze's general Galois theory is a description of the
coverings of B - or rather of those made trivial by pullback along a GIVEN
effective descent map p: E --> B - as the actions on X of a certain Galois
pregroupoid of the extension (E,p).
It is often, but not always, the case that the category of coverings of B is
reflective in the category of all S-maps into B. The aim of the present talk
is to give sufficient conditions for this, and to prove them satisfied in each
of the important examples above.