Omari, Pierpaolo et Zanolin, Fabio, A Note on Nonlinear Oscillations at Resonance, Acta Mathematica Sinica, New Series Chinese Journal of Mathematics, 3(4), 1987, pp. 351-361
Existence of 2.-periodic solutions to the equation x + g(x)=p(t) is proved, under sharp nonresonance conditions on the interaction of g(x)/x with two consecutive eigenvalues m² and (m²+1): touch with the eigenvalues is allowed.