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Motivated by constructions in the theory of inverse semigroups and etale groupoids, we define and investigate the concept of isotropy from a topos-theoretic perspective. Our main conceptual tool is a monad on the category of grouped toposes. Its algebras correspond to a generalized notion of crossed module, which we call a crossed topos. As an application, we present a topos-theoretic characterization and generalization of the `Clifford, fundamental' sequence associated with an inverse semigroup.
@article{TAC_2012_26_a23, author = {Jonathon Funk and Pieter Hofstra and Benjamin Steinberg}, title = {Isotropy and crossed toposes}, journal = {Theory and applications of categories}, pages = {660--709}, publisher = {mathdoc}, volume = {26}, year = {2012}, language = {en}, url = {http://geodesic.mathdoc.fr/item/TAC_2012_26_a23/} }
Jonathon Funk; Pieter Hofstra; Benjamin Steinberg. Isotropy and crossed toposes. Theory and applications of categories, Tome 26 (2012), pp. 660-709. http://geodesic.mathdoc.fr/item/TAC_2012_26_a23/