1. M. Barr, Exact Categories, Lecture Notes in Mathematics, Vol. 236, Springer, Berlin, 1971, pp. 1–120.
2. The tower of n-groupoids and the long cohomology sequence;Bourn;J. Pure Appl. Algebra,1989
3. D. Bourn, Normalization Equivalence, Kernel Equivalence, and Affine Categories, Lecture Notes in Mathematics, Vol. 1488, Springer, Berlin, 1991, pp. 43–62.
4. Baer sums and fibred aspects of Mal'cev operations;Bourn;Cahiers Topologie Géom. Differentieile Categoriques,1999
5. Normal subobjects and abelian objects in protomodular categories;Bourn;J. Algebra,2000