*Published Paper*

**Inserted:** 12 dec 2013

**Last Updated:** 27 sep 2015

**Journal:** ARMA

**Year:** 2014

**Doi:** 10.1007/s00205-014-0801-6

**Links:**
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**Abstract:**

We study the optimal sets $\Omega^\ast\subset\mathbb{R}^d$ for spectral functionals $F\big(\lambda_1(\Omega),\dots,\lambda_p(\Omega)\big)$, which are bi-Lipschitz with respect to each of the eigenvalues $\lambda_1(\Omega),\dots,\lambda_p(\Omega)$ of the Dirichlet Laplacian on $\Omega$, a prototype being the problem

$\qquad\qquad\qquad\qquad\qquad\qquad\qquad\qquad\qquad \min{\big\{\lambda_1(\Omega)+\dots+ \lambda_p(\Omega)\;:\;\Omega\subset\mathbb{R}^d,\

\Omega

=1\big\}}. $

We prove the Lipschitz regularity of the eigenfunctions $u_1,\dots,u_p$ of the Dirichlet Laplacian on the optimal set $\Omega^*$ and, as a corollary, we deduce that $\Omega^*$ is open.

For functionals depending only on a generic subset of the spectrum, as for example $\lambda_k(\Omega)$ or $\lambda_{k_1}(\Omega)+\dots+\lambda_{k_p}(\Omega)$ , our result proves only the existence of a Lipschitz continuous eigenfunction in correspondence to each of the eigenvalues involved.

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