1. Abstract and Concrete Categories: The Joy of Cats;Adámek,2006
2. Éléments de mathématique. Topologie générale. Chapitres 1 à 4;Bourbaki,1971
3. Foundations of General Topology;Császár,1963
4. Closure operators I;Dikranjan;Topol. Appl.,1987
5. Categorical Structure of Closure Operators with Applications to Topology, Algebra and Discrete Mathematics;Dikrajan,1995