1. Absolute retracts in group theory;Baer;Bull. Am. Math. Soc.,1946
2. Exact categories;Barr,1971
3. Mal'cev, Protomodular, Homological and Semi-Abelian Categories;Borceux,2004
4. Split Extension Classifier and Centrality;Borceux,2007
5. Initial normal covers in bi-Heyting toposes;Borceux;Arch. Math.,2006