Polygonal Faber-Krahn inequality: local minimality via validated computing

Abstract

The main result of the paper shows that the regular [Formula: see text]-gon is a local minimizer for the first Dirichlet-Laplace eigenvalue among [Formula: see text]-gons having fixed area for [Formula: see text]. The eigenvalue is seen as a function of the coordinates of the vertices in [Formula: see text]. Relying on fine regularity results of the first eigenfunction in a convex polygon, an explicit a priori estimate is given for the eigenvalues of the Hessian matrix associated to the discrete problem, whose coefficients involve the solutions of some Poisson equations with singular right hand sides. The a priori estimates, in conjunction with certified finite element approximations of these singular PDEs imply the local minimality for [Formula: see text]. All computations, including the finite element computations, are realized using interval arithmetic.