If is a small category (or even a topological category), one can define a -torsor (or torsor with structure category ) which generalizes the torsor (principal bundle) with structure group(oid). We present two variants in slightly different context.
If is a sheaf over , denote by its stalk over (cf. etale space).
A -torsor over a topological space is given by a functor such that
(surjectivity) every ‘total stalk’ , where , is nonempty;
(transitivity) for any two germs ‘in the same total stalk’, , , there is a span and such that and ;
(freeness) for a parallel pair of morphisms in , for some implies there is a morphism and such that and .
This definition is from the monograph
where it is shown that the classifying space of a category classifies -torsors.
David Roberts: This definition should be able to be restated in terms of flat functors
Suppose now is a finitely complete category with a calculus of left fractions whose morphisms are called covers.
Let be an internal category in . An -torsor trivialized by a cover is a discrete fibration for which there exist a morphism and a commutative diagram
in which the square is a pullback. Street says -torsor at for an -torsor trivialized by some cover .
Last revised on December 18, 2019 at 00:30:30. See the history of this page for a list of all contributions to it.