1. “Saddle points and multiple solutions of differential equations.”;Amann;Math. Z.,1979
2. Bates, P. W., and Castro, A. (1979). “Necessary and sufficient conditions for existence of solutions to equations with noninvertible linear part.” Preprint.
3. “Hammerstein integral equations with indefinite kernel.”;Castro;Internat. J. Math. and Math. Sci.,1978
4. “Functional analysis and periodic solutions of nonlinear differential equations.”;Cesari,1963
5. “Functional analysis and Galerkin's method.”;Cesari;Mich. Math. J.,1964