1. Notes on extensions of Hopf algebras;Andruskiewitsch;Can. J. Math.,1996
2. Exact Categories and Categories of Sheaves;Barr,1971
3. Mal'cev, Protomodular, Homological and Semi-Abelian Categories;Borceux,2004
4. Internal object actions;Borceux;Comment. Math. Univ. Carol.,2005