1. Theorems of existence and multiplicity for nonlinear elliptic problems with non invertible linear part;Ambrosetti;Annali Scu. norm. sup. Pisa,1978
2. Nontrivial solutions for a class of nonresonance problems and applications to nonlinear differential equations;Amann;Annali Pisa,1980
3. A semilinear Dirichlet problem;Castro;Can. J. Math.,1979
4. Critical point theory and the number of solutions of a nonlinear Dirichlet problem;Castro;Annali Mat. pura appl.,1979
5. Theory of Ordinary Differential Equations;Coddington,1955