[1] S. AHMAD A. C. LAZER J. L. PAUL: Elementary critical point theory and perturbations of elliptic boundary value problems at resonance. Indiana Univ. Math. Journal 25 (1976), 933-944. MR 0427825

[2] M. S. BERGER M. SCHECHTER: On the solvability of semilinear operator equations and elliptic boundary value problems. Bull. Amer. Math. Soc. 78 (1972), 741-745. MR 0303374

[3] S. FUČÍK: Nonlinear potential equations with linear parts at resonance. (to appear). MR 0482425

[4] S. FUČÍK J. NEČAS V. SOUČEK: Variationsrechnung. Teubner Texte zur Mathematik, Teubner, Leipzig, 1977. MR 0487654

[5] A. C. LAZER E. M. LANDESMAN D. R. MEYERS: On saddle point problems in the calculus of variations, the Ritz algorithm, and monotone convergence. J. Math. Anal. Appl. 52 (1975), 594-614. MR 0420389

[6] A. C. LAZER: Some resonance problems for elliptic boundary value problems. Lecture Notes in Pure and Applied Mathematics No 19: Nonlinear Functional Analysis and Differential Equations (ed.: L. Cesari, R. Kannan, J.D. Schuur), pp. 269-289. M. Dekker Inc., New York and Basel, 1976. MR 0487005

[7] M. M. VAJNBERG: Variational methods for the study of nonlinear operators. (Russian), Moscow 1956. English transl.: Holden-Day, San Francisco, California 1964. MR 0176364