Optimal shapes for the first Dirichlet eigenvalue of the p-Laplacian and dihedral symmetry

Abstract

In this paper, we consider the optimization problem for the first Dirichlet eigenvalue λ1(Ω) of the p-Laplacian Δp, 1<p<∞, over a family of doubly connected planar domains Ω=B∖P‾, where B is an open disk and P⊊B is a domain which is invariant under the action of a dihedral group Dn for some n≥2,n∈N. We study the behaviour of λ1 with respect to the rotations of P about its centre. We prove that the extremal configurations correspond to the cases where Ω is symmetric with respect to the line containing both the centres. Among these optimizing domains, the OFF configurations correspond to the minimizing ones while the ON configurations correspond to the maximizing ones. Furthermore, we obtain symmetry (periodicity) and monotonicity properties of λ1 with respect to these rotations. In particular, we prove that the conjecture formulated in [11] for n odd and p=2 holds true. As a consequence of our monotonicity results, we show that if the nodal set of a second eigenfunction of the p-Laplacian possesses a dihedral symmetry of the same order as that of P, then it can not enclose P.