[1] Sh. Ahmad: An existence theorem for periodically perturbed conservative systems. Michigan Math. J. 20 (1973) 385-392. MR 0346246 | Zbl 0294.34029

[2] S. Banach: Sur les opérations dans les ensembles abstraits et leur application aux équations intégrales. Fundam. Math. 3 (1922) 133-181.

[3] L. Cesari: Alternative methods in nonlinear analysis. Proc. Int. Conf. Differential Equations, Los Angeles, Sept. 74, Academic Press, 1975, 95-148. MR 0430884 | Zbl 0316.47039

[4] C. L. Dolph: Non-linear integral equations of the Hammerstein type. Trans. Amer. Math. Soc. 66 (1949) 289-307. MR 0032923

[5] H. Ehrmann : On implicit function theorems and the existence of solutions of non-linear equations. L'Enseignement Math. 9 (1963) 129-176. MR 0156175 | Zbl 0127.08004

[6] H. Ehrmann, H. E. Lahmann: Anwendungen des Schauderschen Fixpunktsatzes auf gewisse nichtlineare Integralgleichungen. L'Enseignement Math. 11 (1965) 267-280. MR 0202013 | Zbl 0196.13002

[7] M. Golomb: Zur Theorie der nichtlinearen Integralgleichungen, Integralgleichungssysteme und allgemeinen Funktionalgleichungen. Math. Z. 39 (1935) 45-75. MR 1545488

[8] J. K. Hale: Applications of alternative problems. Brown Univ. Lecture Notes 71-1, Providence, RI, 1971.

[9] A. Hammerstein: Nichtlineare Integralgleichungen nebst Anwendungen. Acta Math. 54 (1930) 117-176. MR 1555304

[10] R. Kannan: Periodically perturbed conservative systems. J. Differential Equations, 16 (1974) 506-514. MR 0417493 | Zbl 0349.34029

[11] T. Kato: Perturbation theory for linear operators. Springer, Berlin, 1966. Zbl 0148.12601

[12] I. Kolodner: Equations of Hammerstein type in Hilbert spaces. J. Math. Mech. 13 (1964) 701-750. MR 0171184 | Zbl 0149.36401

[13] A. C. Lazer, D. A. Sanchez: On periodically perturbed conservative systems. Michigan Math. J. 16 (1969) 193-200. MR 0245906 | Zbl 0187.34501

[14] A. C. Lazer: Application of a lemma on bilinear forms to a problem in nonlinear oscillations. Proc. Amer. Math. Soc. 33 (1972) 89-94. MR 0293179 | Zbl 0257.34041

[15] D. E. Leach: On Poincaré's perturbation theorem and a theorem of W. S. Loud. J. Differential Equations 7 (1970) 34-53. MR 0251308 | Zbl 0186.15501

[16] W. S. Loud: Periodic solutions of nonlinear differential equations of Duffing type. Proc. U.S.-Japan Semin. on Diff. and Funct. Equations, Benjamin, New York, 1967, 199-224. MR 0223656 | Zbl 0162.12302

[17] N. Rouche, J. Mawhin: Equations différentielles ordinaires. vol. 1, Masson, Paris, 1973. MR 0481182 | Zbl 0289.34001

[18] M. M. Vainberg: Variational method and method of monotone operators in the theory of nonlinear equations. Wiley, New York, 1973. MR 0467428 | Zbl 0279.47022