1. Topologie Générale, ch. I et II;Bourbaki,1961
2. Elements of Modern Topology;Brown,1968
3. Topology and Groupoids;Brown,2006
4. Van Kampen theorems for categories of covering morphisms in lextensive categories;Brown;J. Pure Appl. Algebra,1997
5. Introduction to extensive and distributive categories;Carboni;J. Pure Appl. Algebra,1993