[1] P. ERDÖS J. SPENCER: 
Probabilistic methods in combinatorics. Akad. Kiado Budapest and Academic Press, New York and London, 1974. 
MR 0382007[2] E. L. LAWLER: 
Combinatorial Optimisation: Networks and Matroids. Holt, Reinhart and Winston 1976. 
MR 0439106[3] A. LUBOTZKY R. PHILLIPS P. SARNAK: Explicit expanders and the Ramanujan conjecture. 18. ACM Annual Symp. on Theory of Comput., 1986, pp. 240-246.
[4] N. ALON V. D. MILMAN: 
${\lambda}_1$-isoperimetric inequalities for graphs and superconcentrators. J. Comb. Theory Ser. B 38 (1985),  pp. 73-88. 
MR 0782626