Shape derivatives for minima of integral functionals
For $Ω$ varying among open bounded sets in ${\mathbb R} ^n$, we consider shape functionals $J (Ω)$ defined as the infimum over a Sobolev space of an integral energy of the kind $\int _Ω[ f (\nabla u) + g (u) ]$, under Dirichlet or Neumann conditions on $\partial Ω$. Under fairly weak assumptions on the integrands $f$ and $g$, we prove that, when a given domain $Ω$ is deformed into a one-parameter family of domains $Ω_\varepsilon$ through an initial velocity field $V\in W ^ {1, \infty} ({\mathbb R} ^n, {\mathbb R} ^n)$, the corresponding shape derivative of $J$ at $Ω$ in the direction of $V$ exists. Under some further regularity assumptions, we show that the shape derivative can be represented as a boundary integral depending linearly on the normal component of $V$ on $\partial Ω$. Our approach to obtain the shape derivative is new, and it is based on the joint use of Convex Analysis and Gamma-convergence techniques. It allows to deduce, as a companion result, optimality conditions in the form of conservation laws.