Re: gr-stacks in the literature

Thanks for replies. I am familiar with Breen's work, and the correspondence between crossed modules and gr-stacks. I ws thinking more along the lines of "a smooth gr- chart K (or whatever it is called) for a differentiable gr-stack G is a representable surjection K --> G satisfying the properties..." (with some 2- cartesian squares following). Such a thing might be in Champs Algebriques , in the category of schemes, but I haven't got it at the library. -- David Roberts Pure Mathematics University of Adelaide South Australia, 5005 You know we all became mathematicians for the same reason: we were lazy. -Max Rosenlicht(1949)