Integral representations of lower semicontinuous envelopes and Lavrentiev Phenomenon for non continuous Lagrangians

Abstract: We consider the functional $$F_\infty(u)=\int_{\Omega}f(x,u(x),\nabla u(x)) dx \quad\quad u\in \varphi+ W_0^{1,\infty}(\Omega,\mathbb{R})$$ where $\Omega$ is an open bounded Lipschitz subset of $\mathbb{R}^N$ and $\varphi\in W^{1,\infty}(\Omega)$. We do not assume neither convexity or continuity of the Lagrangian w.r.t. the last variable. We prove that, under suitable assumptions, the lower semicontinuous envelope of $F_\infty$ both in $\varphi+W^{1,\infty}(\Omega)$ and in the larger space $\varphi+W^{1,p}(\Omega)$ can be represented by means of the bipolar $f^{**}$ of $f$. In particular we can also exclude Lavrentiev Phenomenon between $W^{1,\infty}(\Omega)$ and $W^{1,1}(\Omega)$ for autonomous Lagrangians.