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arXiv · 2609.17083

On some functionals for which the ball is a saddle shape

Alba Lia Masiello·Gloria Paoli·Francesco Salerno
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Abstract

In the present paper, we study the maximization problem of the $k$-Torsional rigidity under quermassintegral constraint, with particular emphasis on the role of the ball. Our main result shows a phenomenon which, as far as we know, has not previously been observed: the ball, rather than being an extremal shape, is a saddle point. More precisely, for suitable quermassintegral constraints, we show that the $k$-torsional rigidity admits both increasing and decreasing perturbations around the ball. The proof is based on a second-order analysis of the corresponding variational problem. To the best of our knowledge, this is the first use of a second-order shape derivative approach in the study of variational problems for $k$-Hessian operators. We also establish an analogous saddle-point behavior for a related curvature functional under a volume constraint, highlighting a structural distinction between the intermediate Hessian regime and the classical Laplace and Monge-Ampère cases.

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Alba Lia Masiello, Gloria Paoli, Francesco Salerno. 2026-09-15. On some functionals for which the ball is a saddle shape. https://arxiv.org/abs/2609.17083

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