[1] A. Ya. DUBOVITSKII, A. A. MILYUTIN: 
Extremum Problems in the presence of restrictions. Zh. Vychisl. Mat. i Fis., Vol. 5, No 3 (1965), 395-453. 
Zbl 0158.33504[4] M. M. VAINBERG: 
Variational methods for the study of nonlinear operators. Holden-Day, Inc., San Francisco, London, Amsterdam, 1964. 
MR 0176364 | 
Zbl 0122.35501[5] V. F. DEM'YANOV, A. M. RUBINOV: 
Minimization of functionals in normed spaces. SIAM J. Control, Vol. 6, No 1 (1968), 73-88. 
MR 0231260 | 
Zbl 0165.49902[6] V. F. DEM'YANOV, A. M. RUBINOV: Approximate methods for extremum problems. University of Leningrad, 1968.
[7] A. D. IOFFE, V. M. TIKHOMIROV: 
Duality of convex functions and extremum problems. Uspekhi Mat. Nauk, Vol. XXIII, No 6 (1968), 55-116. 
Zbl 0191.13101[8] A. Ya. DUBOVITSKII, A. A. MILYUTIN: Translation of Euler's equations. Zh. Vychisl. Mat. i Mat. Fiz.,Vol. 9, No 6 (1969), 1263-1284.