Complete spectrum of the Robin eigenvalue problem on the ball (2510.26331v1)

Abstract: We investigate the following Robin eigenvalue problem \begin{equation*} \left{ \begin{array}{ll} -\Delta u=\mu u\,\, &\text{in}\,\, B,\ \partial_\texttt{n} u+\alpha u=0 &\text{on}\,\, \partial B \end{array} \right. \end{equation*} on the unit ball of $\mathbb{R}^N$. We obtain the complete spectral structure of this problem. In particular, for $\alpha>0$, we find that the first eigenvalue is $k_{\nu,1}^2$ and the second eigenvalue is exactly $k_{\nu+1,1}^2$, where $k_{\nu+l,m}$ is the $m$th positive zero of $kJ_{\nu+l+1}(k)-(\alpha+l) J_{\nu+l}(k)$. Moreover, when $\alpha\in(-1,0)$, the first eigenvalue is $-\widehat{k}{\nu,1}^2$ where $\widehat{k}{\nu,1}$ denotes the unique zero of $\alpha I_{\nu}(k)+kI_{\nu+1}(k)$, and the second eigenvalue is exactly $k_{\nu+1,1}^2$. Furthermore, for $\alpha=-1$, the first eigenvalue is $-\widehat{k}_{\nu,1}^2$ and the second eigenvalue is exactly $0$. Our conclusions indicate the ratio $\mu_2/\mu_1$ may be positive, negative or zero according to the suitable ranges of the parameter $\alpha$.