Non-local Functionals Related to the Total Variation and Connections with Image Processing

We present new results concerning the approximation of the total variation, $\int_{\Omega} |\nabla u|$, of a function $u$ by non-local, non-convex functionals of the form $\Lambda_\delta u = \int_{\Omega} \int_{\Omega} \frac{\delta \varphi \big( |u(x) - u(y)|/ \delta\big)}{|x - y|^{d+1}} \, dx \, dy,$ as $\delta \to 0$, where $\Omega$ is a domain in $\mathrm{R}^d$ and $\varphi: [0, + \infty) \to [0, + \infty)$ is a non-decreasing function satisfying some appropriate conditions. The mode of convergence is extremely delicate and numerous problems remain open. De Giorgi's concept of Gamma-convergence illuminates the situation, but also introduces mysterious novelties. The original motivation of our work comes from Image Processing.