[1] R. C. BOSE S. S. SHRIKHANDE: 
Graphs in which each paiг of veгtices is adjacent to the same number d of other vertices. Studia Sci. Math. Hung., 5, 1970, 181-195. 
MR 0274319 
[2] F. HARARY: 
Gгaph theoгy. Addison-Wesley, Reading (Mass.) 1969. 
MR 0256911 
[3] G. HIGMAN: Lectuгe given at the Confeгence on combinatorial analysis. Oxford Univeгsity, 1969.
[4] J. PLESNÍK: 
Graphs with a homogeneity. Glasnik Matematički, 10 (30), 1975, 9-23. 
MR 0398905 
[5] H. S. WILF: 
The friendship theoгem. In: Comb. Math. and its Applications, (D. J. A. Welsh, ed.), Academic Press, London, 1971, 307-309. 
MR 0282857