1. Ambrosetti, A.: On the existence of multiple solutions for a class of nonlinear boundary value problems. Rend. Sem. Mat. Univ. Padova 49, 195–204 (1973)
2. Ambrosetti, A.: A perturbation theorem for superlinear boundary value problems. Tech. Sum. Report 1446, Wisconsin Univ Madison Mathematics Research Center (1974)
3. Ambrosetti, A., Rabinowitz, P.H.: Dual variational methods in critical point theory and applications. J. Funct. Anal. 14, 349–381 (1973)
4. Bahri, A.: Topological results on a certain class of functionals and application. J. Funct. Anal. 41(3), 397–427 (1981)
5. Bahri, A., Berestycki, H.: Points critiques de perturbations de fonctionnelles paires et applications. C. R. Acad. Sci. Paris Sér A–B 291(3), A189–A192 (1980)