Multiplicity theorems involving functions with non-convex range

Abstract

"Here is a sample of the results proved in this paper: Let $f:{\bf R}\to {\bf R}$ be a continuous function, let $\rho>0$ and let $\omega:[0,\rho[\to [0,+\infty[$ be a continuous increasing function such that $$\lim\limits_{\xi\to \rho^-}\ds\int_0^{\xi}\omega(x)dx=+\infty.$$ Consider $C^0([0,1])\times C^0([0,1])$ endowed with the norm $$\|(\alpha,\beta)\|=\int_0^1|\alpha(t)|dt+\int_0^1|\beta(t)|dt.$$ Then, the following assertions are equivalent: \noindent $(a)$ the restriction of $f$ to $\left [-{{\sqrt{\rho}}\over {2}},{{\sqrt{\rho}}\over {2}} \right ]$ is not constant; \noindent $(b)$ for every convex set $S\subseteq C^0([0,1])\times C^0([0,1])$ dense in $C^0([0,1])\times C^0([0,1])$, there exists $(\alpha,\beta)\in S$ such that the problem $$\left\{\begin{array}{l} -\omega\left(\displaystyle\int_0^1|u'(t)|^2dt\right)u'' =\beta(t)f(u)+\alpha(t) \mbox{ in } [0,1]\\ u(0)=u(1)=0\\ \displaystyle\int_0^1|u'(t)|^2dt<\rho \end{array}\right.$$ has at least two classical solutions."