“…Another interesting work in the same direction is due to Bonder, Groisman and Rossi, who studied the so called Sobolev trace inequality (see [9,20]), thus they were interested in the optimization of the first nonzero eigenvalue of an elliptic operator with mixed Dirichlet-Steklov boundary conditions among perforated domains: the existence and regularity of an optimal hole are proved in [22,23], and by using shape derivatives it is shown that annulus is a critical but not an optimum shape (see [22]). At last, we point out the recent papers [26,36,41,46,48], where the authors consider the first eigenvalue of the Laplace operator with mixed Dirichlet-Steklov boundary conditions. Many examples stated in the last paragraph deal with linear operators eigenvalues in the special case of doubly connected domains with spherical outer and inner boundaries.…”