1. L. Amaral andM. P. Pera, A note on periodic solutions at resonance. Boll. Un. Mat. Ital. C(5)18, 107?117 (1981).
2. L. Cesari andR. Kannan, Solutions in the large of Liénard equations with forcing terms. Ann. Mat. Pura Appl. (4)111, 101?124 (1976).
3. C. P. Gupta, On functional equations of Fredholm and Hammerstein type with applications to existence of periodic solutions of certain ordinary differential equations. J. Integral Equations (1)3, 21?42 (1981).
4. C. P. Gupta andJ. Mawhin, Asymptotic conditions at the two first eigenvalues for the periodic solutions of Liénard differential equations and an inequality of E. Schmidt. Z. Anal. Anwendungen (1)3, 33?42 (1984).
5. P. Habets andM. N. Nkashama, On periodic solutions of non-linear second order vector differential equations. Sém. Math. (Nouv. Sér.) Univ. Catholique Louvain68, IX-1?24 (1985).