Weights sharing the same eigenvalue

Here is the simplest particular case of our main result: let $f:{\bf R}\to {\bf R}$ be a function of class $C^1$, with $\sup_{\bf R}f'>0$, such that $$\lim_{|\xi|\to +\infty}{{f(\xi)}\over {\xi}}=0\ .$$ Then, for each $\lambda>{{\pi^2}\over {\sup_{\bf R}f'}}$, the set of all $u\in H^1_0(]0,1[)$ for which the problem \cases{-v''=\lambda f'(u(x)) v & in $]0,1[$\cr & \cr v(0)=v(1)=0\cr} has a non-zero solution is closed and not $\sigma$-compact in $H^1_0(]0,1[)$.