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Any semi-abelian category A appears, via the discrete functor, as a full replete reflective subcategory of the semi-abelian category of internal groupoids in A. This allows one to study the homology of $n$-fold internal groupoids with coefficients in a semi-abelian category A, and to compute explicit higher Hopf formulae. The crucial concept making such computations possible is the notion of protoadditive functor, which can be seen as a natural generalisation of the notion of additive functor.
@article{TAC_2010_23_a1, author = {Tomas Everaert and Marino Gran}, title = {Homology of n-fold groupoids}, journal = {Theory and applications of categories}, pages = {22--41}, publisher = {mathdoc}, volume = {23}, year = {2010}, language = {en}, url = {http://geodesic.mathdoc.fr/item/TAC_2010_23_a1/} }
Tomas Everaert; Marino Gran. Homology of n-fold groupoids. Theory and applications of categories, The Bourn Festschrift, Tome 23 (2010), pp. 22-41. http://geodesic.mathdoc.fr/item/TAC_2010_23_a1/