  Let us consider the following minimum problem \[ \lambda_\alpha(p,r)=
\min_{\substack{u\in W_{0}^{1,p}(-1,1)\\
u\not\equiv0}}\dfrac{\displaystyle\int_{-1}^{1}|u'|^{p}dx+\alpha\left|\int_{-1}^{1}|u|^{r-1}u\,
dx\right|^{\frac pr}}{\displaystyle\int_{-1}^{1}|u|^{p}dx}, \] where
$\alpha\in\mathbb R$, $p\ge 2$ and $\frac p2 \le r \le p$. We show that there
exists a critical value $\alpha_C=\alpha_C (p,r)$ such that the minimizers have
constant sign up to $\alpha=\alpha_{C}$ and then they are odd when
$\alpha>\alpha_{C}$.
