  Let $\Omega_0$ denote the unit ball of $\mathbb{R}^N$ ($N\ge 2$) centered at
the origin. We suppose that $\Omega_0$ contains a core, given by a smaller
concentric ball $D_0$, made of a (possibly) different material. We discover
that, depending on the relative hardness of the two materials, this radial
configuration can either be a local maximizer for the torsional rigidity
functional $E$ or a saddle shape. In this paper we consider perturbations that
simultaneously act on the boundaries $\partial D_0$ and $\partial\Omega_0$.
This gives rise to resonance effects that are not present when $\partial D_0$
or $\partial\Omega_0$ are perturbed in isolation. A detailed analysis of the
sign of the second order shape derivative of $E$ is then made possible by
employing the use of spherical harmonics.

Analysis of PDEs