  We consider a spectral stability estimate by Burenkov and Lamberti concerning
the variation of the eigenvalues of second order uniformly elliptic operators
on variable open sets in the N-dimensional euclidean space, and we prove that
it is sharp for any dimension N. This is done by studying the eigenvalue
problem for the Dirichlet Laplacian on special open sets inscribed in suitable
spherical cones.
