A sharp centro-affine isospectral inequality of Szeg\"{o}--Weinberger type and the $L^p$-Minkowski problem.

We establish a sharp upper-bound for the first non-zero even eigenvalue (corresponding to an even eigenfunction) of the Hilbert-Brunn-Minkowski operator associated to a strongly convex $C^2$-smooth origin-symmetric convex body $K$ in $\mathbb{R}^n$. Our isospectral inequality is centro-affine invariant, attaining equality if and only if $K$ is a (centered) ellipsoid; this is reminiscent of the (non affine invariant) classical Szeg\"{o}--Weinberger isospectral inequality for the Neumann Laplacian. The new upper-bound complements the conjectural lower-bound, which has been shown to be equivalent to the log-Brunn-Minkowski inequality and is intimately related to the uniqueness question in the even log-Minkowski problem. As applications, we obtain new strong non-uniqueness results in the even $L^p$-Minkowski problem in the subcritical range $-n < p < 0$, as well as new rigidity results for the critical exponent $p=-n$ and supercritical regime $p < -n$. In particular, we show that any $K$ as above which is not an ellipsoid is a witness to non-uniqueness in the even $L^p$-Minkowski problem for all $p \in (-n,p_K)$ and some $p_K \in (-n,0)$, and that $K$ can be chosen so that $p_K$ is arbitrarily close to $0$.