Discarding Lavrentiev's Gap in Non-automonous and Non-Convex Variational Problems

Abstract: We establish that the Lavrentiev gap between Sobolev and Lipschitz maps does not occur for a scalar variational problem of the form: [ \textrm{to minimize} \qquad u \mapsto \int_\Omega f(x,u,\nabla u)\text{d}x \,, ] under a Dirichlet boundary condition. Here, $\Omega$ is a bounded Lipschitz open set in $\mathbb{R}^N$, $N\geq 1$ and the function $f$ is required to be measurable with respect to the spacial variable, continuous with respect to the second one, and continuous and comparable to convex with respect to the last variable. Moreover, we assume that $f$ satisfies a natural condition balancing the variations with respect to the first variable and the growth with respect to the last one. Remarkably, typical conditions that are usually imposed on $f$ to discard the Lavrentiev gap are dropped here: we do not require $f$ to be bounded or convex with respect to the second variable, nor impose any condition of $\Delta_2$-kind with respect to the last variable.