Global calibrations for the non-homogeneous Mumford-Shah functional

Using a calibration method we prove that, if $\Gamma\subset \Omega$ is a closed regular hypersurface and if the function $g$ is discontinuous along $\Gamma$ and regular outside, then the function $u_{\beta}$ which solves $$\begin{cases} \Delta u_{\beta}=\beta(u_{\beta}-g)& \text{in $\Omega\setminus\Gamma$} \partial_{\nu} u_{\beta}=0 & \text{on $\partial\Omega\cup\Gamma$} \end{cases}$$ is in turn discontinuous along $\Gamma$ and it is the unique absolute minimizer of the non-homogeneous Mumford-Shah functional $$\int_{\Omega\setminus S_u}|\nabla u|^2 dx +{\cal H}^{n-1}(S_u)+\beta\int_{\Omega\setminus S_u}(u-g)^2 dx,$$ over $SBV(\Omega)$, for $\beta$ large enough. Applications of the result to the study of the gradient flow by the method of minimizing movements are shown.Comment: 33 page

Cascade of minimizers for a nonlocal isoperimetric problem in thin domains

For \Omega_\e=(0,\e)\times (0,1) a thin rectangle, we consider minimization of the two-dimensional nonlocal isoperimetric problem given by \inf_u E^{\gamma}_{\Omega_\e}(u) where E^{\gamma}_{\Omega_\e}(u):= P_{\Omega_\e}(\{u(x)=1\})+\gamma\int_{\Omega_\e}\abs{\nabla{v}}^2\,dx and the minimization is taken over competitors u\in BV(\Omega_\e;\{\pm 1\}) satisfying a mass constraint \fint_{\Omega_\e}u=m for some $m\in (-1,1)$. Here P_{\Omega_\e}(\{u(x)=1\}) denotes the perimeter of the set $\{u(x)=1\}$ in \Omega_\e, \fint denotes the integral average and $v$ denotes the solution to the Poisson problem -\Delta v=u-m\;\mbox{in}\;\Omega_\e,\quad\nabla v\cdot n_{\partial\Omega_\e}=0\;\mbox{on}\;\partial\Omega_\e,\quad\int_{\Omega_\e}v=0. We show that a striped pattern is the minimizer for \e\ll 1 with the number of stripes growing like $\gamma^{1/3}$ as $\gamma\to\infty.$ We then present generalizations of this result to higher dimensions.Comment: 20 pages, 2 figure

Functionals depending on curvatures with constraints

We deal with a family of functionals depending on curvatures and we prove for them compactness and semicontinuity properties in the class of closed and bounded sets which satisfy a uniform exterior and interior sphere condition. We apply the results to state an existence theorem for the Nitzberg and Mumford problem under this additional constraint.Comment: 20 pages. To appear on Rendiconti del Seminario Matematico dell'Universita' di Padov

Local calibrations for minimizers of the Mumford-Shah functional with a regular discontinuity set

Using a calibration method, we prove that, if w is a function which satisfies all Euler conditions for the Mumford-Shah functional on a two-dimensional domain, and the discontinuity set S of w is a regular curve connecting two boundary points, then there exists a uniform neighbourhood U of S such that w is a minimizer of the Mumford-Shah functional on U with respect to its own boundary conditions. We show that Euler conditions do not guarantee in general the minimality of w in the class of functions with the same boundary value of w and whose extended graph is contained in a neighbourhood of the extended graph of w, and we give a sufficient condition in terms of the geometrical properties of the domain and the discontinuity set under which this kind of minimality holds.Comment: 31 pages, 2 figure

Nonlocal curvature flows

This paper aims at building a unified framework to deal with a wide class of local and nonlocal translation-invariant geometric flows. First, we introduce a class of generalized curvatures, and prove the existence and uniqueness for the level set formulation of the corresponding geometric flows. We then introduce a class of generalized perimeters, whose first variation is an admissible generalized curvature. Within this class, we implement a minimizing movements scheme and we prove that it approximates the viscosity solution of the corresponding level set PDE. We also describe several examples and applications. Besides recovering and presenting in a unified way existence, uniqueness, and approximation results for several geometric motions already studied and scattered in the literature, the theory developed in this paper allows us to establish also new results

Minimality via second variation for a nonlocal isoperimetric problem

We discuss the local minimality of certain configurations for a nonlocal isoperimetric problem used to model microphase separation in diblock copolymer melts. We show that critical configurations with positive second variation are local minimizers of the nonlocal area functional and, in fact, satisfy a quantitative isoperimetric inequality with respect to sets that are $L^1$-close. The link with local minimizers for the diffuse-interface Ohta-Kawasaki energy is also discussed. As a byproduct of the quantitative estimate, we get new results concerning periodic local minimizers of the area functional and a proof, via second variation, of the sharp quantitative isoperimetric inequality in the standard Euclidean case. As a further application, we address the global and local minimality of certain lamellar configurations.Comment: 35 page

Local calibrations for minimizers of the Mumford-Shah functional with rectilinear discontinuity sets

Using a calibration method, we prove that, if $w$ is a function which satisfies all Euler conditions for the Mumford-Shah functional on a two-dimensional open set $\Omega$, and the discontinuity set of $w$ is a segment connecting two boundary points, then for every point $(x_0, y_0)$ of $\Omega$ there exists a neighbourhood $U$ of $(x_0, y_0)$ such that $w$ is a minimizer of the Mumford-Shah functional on $U$ with respect to its own boundary values on $\partial U$.Comment: 22 pages, 4 figure

Motion of three-dimensional elastic films by anisotropic surface diffusion with curvature regularization

Short time existence for a surface diffusion evolution equation with curvature regularization is proved in the context of epitaxially strained three-dimensional films. This is achieved by implementing a minimizing movement scheme, which is hinged on the $H^{-1}$-gradient flow structure underpinning the evolution law. Long-time behavior and Liapunov stability in the case of initial data close to a flat configuration are also addressed.Comment: 44 page