[2] M. BERGER: Non linearity and functional Analysis. Academic Press N.Y., 1977.
[3] P. HESS: On nontrivial solutions of a non linear elliptic boundary value problems. 173 Conferenze del Seminario di Matematica dell' Universita di Bari, 1980.
[4] M. A. KRASNOSEL'SKII:
Topological Methods in the Theory of non linear Integral Equations. Macmillan Company, N.Y., 1964.
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