1. Graphs in which each pair of vertices is adjacent to the same number d of other vertices;Bose;Studia Sci. Math. Hungar.,1970
2. On a problem of graph theory;Erdös;Stud. Sci. Math. Hung.,1967
3. The friendship theorem;Longyear;Indagationes Math.,1972
4. The friendship theorem;Wilf,1971