[1] Gupta C. P.: On Functional Equations of Fred hoi in and Hammerstein type with Application to Existence of Periodic Solutions of Certain Ordinary Differential Equations. Journal of Integral Equations 3 (1981), 21-41. MR 0604314

[2] Gupta C. P., Mawhin J.: Asymptotic Conditions at the First two Eigenvalues for the Periodic Solutions of Lienard Differential Equations and an Inequality of E. Schmidt. Z. Anal. Anwendnngen 3 (1984), 33-42. MR 0739844

[3] Gupta C. P., Nieto J. J., Sanchez L.: Periodic Solutions of Some Lienard and Duffing Equations. Jour. Math. Anal. & Appl. 140 (1989), 67-82. MR 0997843 | Zbl 0689.34032

[4] Loud W. S.: Periodic Solutions of $x" + cx' + g(x) = \epsilon f(t)$. Mem. Amer. Math. Soc., Providence, RI, 1959. MR 0107058

[5] Mawhin J.: Compacitè, Monotonie et Convexitè dans l'etude de problèmes aux limites semilinèaires. Sem. Anal. Moderne Université de Sherbrooke 19 (1981).

[6] Mawhin J.: Landesman-Lazer type Problems for Non-linear Equations. Confer. Sem. Mat. Univ. Bari 147 (1977). MR 0477923

[7] Mawhin J.: Topological Degree Methods in Nonlinear Boundary Value Problems. CBMS Regional Conf. Math. Ser. Math, vol. 40, American Math. Society, Providence, RI, 1979. MR 0525202 | Zbl 0414.34025

[8] Nieto J. J , and Rao V. S. H.: Periodic Solutions for Scalar Lienard Equations. Acta Math. Hung. 57 (1991), 15-27. MR 1128836