Published online by Cambridge University Press: 24 January 2018
Employing the inverse function theorem on Banach spaces, we prove that in a $C^{2}(S^{n-1})$-neighborhood of the unit ball, the only solutions of
$\unicode[STIX]{x1D6F1}^{2}K=cK$ are origin-centered ellipsoids. Here
$K$ is an
$n$-dimensional convex body,
$\unicode[STIX]{x1D6F1}K$ is the projection body of
$K$ and
$\unicode[STIX]{x1D6F1}^{2}K=\unicode[STIX]{x1D6F1}(\unicode[STIX]{x1D6F1}K).$