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Spectral analysis of a generalized buckling problem on a ball

In this paper, the spectrum of the following fourth order problem \begin{equation*} \begin{cases} Δ^2 u+νu=-λΔu &\text{in } D_1,\newline u=\partial_r u= 0 &\text{on } \partial D_1, \end{cases} \end{equation*} where $D_1$ is the unit ball in ${\mathbb R}^N$, is determined for $ν< 0$ as well as the nodal properties of the corresponding eigenfunctions. In particular, we show that the first eigenvalue is simple and that the corresponding eigenfunction is radial and (up to a multiplicative factor) positive and decreasing with respect to the radius. This completes earlier results obtained for $ν\ge 0$ and for $ν<0$.