[1] V. I. Arnold:
Geometric Methods in the Theory of Ordinary Differential Equations. Springer-Verlag, N.Y., 1983.
MR 0695786
[2] J. Carry S.-N. Chow, J. K. Hale:
Abelian integrals and bifurcation theory. J. Diff. Eqn., 59(1985), 413-436.
MR 0807855
[5] W. S. Loud:
Periodic solution of x" + cx + g(x) = ε f(t). Mem. Amer. Math. Soc., No. 31 (1959), 1-57.
MR 0107058
[6] C. Obi:
Analytical theory of nonlinear oscillation, VII, The periods of the periodic solutions of the equation x" + g(x) = 0. J. Math. Anal. Appl. 55 (1976), 295-301.
MR 0460796
[7] Z. Opial:
Sur les periodes des solutions de Pequation differentielle x" + g(x) - 0. Ann. Pol. Math. 10 (1961), 49-72.
MR 0121544
[8] R. Schaaf:
Global behavior of solution branches for some Neumann problems depending on one or several parameters. to appear.
MR 0727393
[9] D. Wang:
On the existence of 2π-periodic solutions of differential equation x" + g(x) = p(t). Chin. Ann. Math., 5A(1) (1984), 61-72.
MR 0743783