Existence of periodic solutions for some singular elliptic equations with strong resonant data

Abstract

We prove the existence of at least one $T$-periodic solution $(T> 0)$ for differential equations of the form $$ \left(\frac{u'(t)}{\sqrt{1-{u'}^2(t)}}\right)' =f(u(t))+h(t),\quad \text{in } (0,T), $$ where $f$ is a continuous function defined on $\mathbb{R}$ that satisfies a {\it strong resonance condition}, $h$ is continuous and with zero mean value. Our method uses variational techniques for nonsmooth functionals.