Existence and regularity of optimal shapes for spectral functionals with Robin boundary conditions

We establish the existence and find some qualitative properties of open sets that minimize functionals of the form $F(\lambda_1(\Omega;\beta),\dots,\lambda_k(\Omega;\beta))$ under measure constraint on $\Omega$, where $\lambda_i(\Omega;\beta)$ designates the $i$-th eigenvalue of the Laplace operator on $\Omega$ with Robin boundary conditions of parameter $\beta>0$. Moreover, we show that minimizers of $\lambda_k(\Omega;\beta)$ for $k\geq 2$ verify the conjecture $\lambda_k(\Omega;\beta)=\lambda_{k-1}(\Omega;\beta)$ in dimension three and more.