A characterization related to a two-point boundary value problem

Abstract

In this short note, we establish the following result: Let $f:[0,+\infty[\to [0,+\infty[$, $\alpha:[0,1]\to ]0,+\infty[$ be two continuous functions, with $f(0)=0$. Assume that, for some $a>0$, the function $\xi\to {{\int_0^{\xi}f(t)dt}\over {\xi^2}}$ is non-increasing in $]0,a]$. Then, the following assertions are equivalent: $(i)$ for each $b>0$, the function $\xi\to {{\int_0^{\xi}f(t)dt}\over {\xi^2}}$ is not constant in $]0,b]$ ; $(ii)$ for each $r>0$, there exists an open interval $I\subseteq ]0,+\infty[$ such that, for every $\lambda\in I$, the problem $$\cases {-u''=\lambda\alpha(t)f(u) & in $[0,1]$\cr & \cr u>0 & in $]0,1[$\cr & \cr u(0)=u(1)=0\cr}$$ has a solution $u$ satisfying $$\int_0^1|u'(t)|^2dt<r\ .$$