We study differential equations with periodic nonlinearities. In particular, we prove that for a small, the equation -u" - u + g(u) = h(t) + a sin(t - phi), where g is a C-1 periodic function with zero mean value and h is orthogonal to cos t and sin t, has 2pi-periodic solutions. The proof relies on an abstract result based on variational arguments; its application makes use of integral estimates by the stationary phase method. We also study the existence of multiple solutions. (C) 2003 Elsevier Ltd. All rights reserved.
Bonheure, D., Fabry, C., & Ruiz, D. (2003). Problems at resonance for equations with periodic nonlinearities. Nonlinear Analysis: Theory, Methods & Applications. https://doi.org/10.1016/j.na.2003.07.005