Multiplicity results for nonlinear elliptic equations. (English). In: Krbec, Miroslav, Kufner, Alois and Rákosník, Jiří (eds.): Nonlinear Analysis, Function Spaces and Applications, Proceedings of the Spring School held in Litomyšl, 1986. Vol. 3. BSB B. G. Teubner Verlagsgesellschaft, Leipzig, 1986. pp. 109-138