1. Exakte und abelsche Kategorien, Korrespondenzen;Brinkmann;Springer Lecture Notes,1969
2. Relations for exact categories (Russian);Calenko;Mat. Sb.,1967
3. Correspondences and exact squares;Hilton,1966
4. Remarks on the axiomatic theory of additive relations;Leicht,1964
5. Natural associativity and commutativity;MacLane;Rice Univ. Studies,1963