[1] H. Amann: Super solutions, monotone iterations, and stability. J. Differential Equations 21 (1976), 363-377. MR 0407451

[2] C. Fabry J. Mawhin, M. N. Nkashama: A multiplicity result for periodic solutions of forced nonlinear second order ordinary differential equations. Bull. London Math. Soc. 18 (1986), 173-180. MR 0818822

[3] P. Hartman: Ordinary Differential Equations. J. Wiley, New York, 1964 (Russian translation, Izdat. Mir, Moskva, 1970). MR 0171038 | Zbl 0125.32102

[4] I. T. Kiguradze, A. G. Lomtatidze: On certain boundary value problems for second-order linear ordinary differential equations with singularities. J. Math. Anal. Appl. 101 (1984), 325-347. MR 0748576 | Zbl 0559.34012

[5] J. Mawhin: Points fixes, points critiques et problèmes aux limites. Sémin. Math. Sup. no. 92, Presses Univ. Montréal, Montréal 1985. MR 0789982 | Zbl 0561.34001

[6] J. Nieto: Nonlinear second-order periodic boundary value problems. J. Math. Anal. Appl. 130 (1988), 22-29. MR 0926825 | Zbl 0678.34022

[7] K. Schmitt: A nonlinear boundary value problem. J. Differential Equations 7 (1970), 527-537. MR 0254314 | Zbl 0198.12301

[8] V. Šeda: Antitone operators and ordinary differential equations. Czech. Math. J. 31 (1981), 531-553. MR 0631601

[9] V. Šeda: A correct problem at a resonance. (preprint). MR 0996746