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Posts Tagged ‘modica-mortola’

## Relaxation of the Anisotropic Perimeter – Part 1

I have discussed in a previous post how Modica-Mortola theorem can provide a good framework for relaxing the perimeter functional in the single and multi-phase cases. The ideas can be extended further to a more generalized notion of perimeter, the anisotropic perimeter. (anisotropic = directionally dependent)

The main idea is that the anisotropic perimeter doesn’t count every part of the boundary in the same way; some directions are more favorized than others. The anisotropic perimeter associated to a norm ${\varphi}$ is defined by

$\displaystyle \text{Per}_\varphi(\Omega)=\int_{\partial \Omega} \varphi(\vec{n})d\mathcal{H}^{n-1}.$

There are variants of Modica-Mortola theorem for the anisotropic perimeter. Here is one of them:

Theorem – Relaxation of the Anisotropic Perimeter

Let ${\Omega}$ be a bounded open set with Lipschitz boundary. Let ${p>1}$, let ${W : \Bbb{R} \rightarrow [0,\infty)}$ be a continuous function such that ${W(z)=0}$ if and only if ${z \in \{0,1\}}$ and let ${\varphi : \Bbb{R}^n \rightarrow [0,\infty)}$ be a norm on ${\Bbb{R}^n}$. Let ${F_\varepsilon : L^1(\Omega) \rightarrow [0,\infty]}$ be defined by

$\displaystyle F_\varepsilon(u) = \begin{cases} \displaystyle \frac{1}{\varepsilon p'}\int_\Omega W(u)dx +\frac{1}{p}\varepsilon^p \int_\Omega \varphi^p(\nabla u)dx & \text{ if } u \in W^{1,p}(\Omega) \\ +\infty & \text{ otherwise} \end{cases}$

and let ${P_\varphi : L^1(\Omega) \rightarrow [0,\infty]}$ be defined by

$\displaystyle P_\varphi(u) =\begin{cases}\displaystyle c_p \int_{S(u)} \varphi(\nu_u)d\mathcal{H}^{n-1}& \text{ if }u \in SBV(\Omega) \text{ and } u \in \{0,1\} \text{ a.e.}\\ +\infty & \text{ otherwise} \end{cases}$

where ${c_p =\int_0^1 (W(s))^{1/p'}ds}$. Then ${\Gamma-\lim_{\varepsilon \rightarrow 0^+}F_\varepsilon(u)=P_\varphi(u)}$.

## Numerical Approximation using Relaxed Formulation

April 23, 2013 1 comment

Sometimes it is easier to replace an optimization problem with a sequence of relaxed problems whose solutions approximate the solution to the initial problem.

This kind of procedure can be useful when we need to approximate numerically discontinuous functions (in particular the characteristic function). Modica Mortola theorem states that the functionals

$\displaystyle F_\varepsilon (u) = \begin{cases} \varepsilon \int_D |\nabla u|^2+\frac{1}{\varepsilon} \int_D W(u) & u \in H^1(D) \\ \infty & \text{ otherwise} \end{cases}$

${\Gamma}$-converges to the functional

$\displaystyle F(u) = \begin{cases} \text{Per}(\{u=1\}) & \text{ if }\{u=1\} \text{ has finite perimeter} \\ \infty & \text{ otherwise} \end{cases}.$

(Recall that ${W}$ is a real function which is positive except for ${0}$ and ${1}$ where it is zero.)

## Master 6

March 3, 2013 1 comment

(For the context see the Shape Optimization page where you can find links to the first 5 parts)

A particular consequence of the Modica-Mortola Theorem is that the functional

$\displaystyle \mathcal{F}(E_1,E_2)=\sigma \text{Per}_\Omega(\partial^* E_1 \cap \partial^* E_2)$

is lower semicontinuous with respect to the ${L^1(\Omega)}$ convergence for ${\sigma>0}$ on the set

$\displaystyle \mathcal{K}=\{ (E_1,E_2) : E_1\cup E_2=\Omega,\ E_1\cap E_2=\emptyset, |E_i|=c_i>0\}$

where the equalities are, as usual, up to a set of measure zero. It would be nice if a similar result would be true for multi-phase systems, where a functional of the form

$\displaystyle \mathcal{F}(E_1,E_2,...,E_k)=\sum_{1\leq i

is a ${\Gamma}$-limit and therefore semicontinuous, for ${E=(E_i) \in \mathcal{K}}$ where

$\displaystyle \mathcal{K}=\{ (E_1,...,E_k) : \bigcup_{i=1}^k E_i=\Omega,\ E_i\cap E_j=\emptyset, \text{ for }i\neq j, |E_i|=c_i>0\}.$

Let’s first remark that allowing the function ${W}$ in the Modica-Mortola theorem to have more than two zeros does not suffice. Indeed, if we allow ${W}$ to have zeros ${\alpha<\beta<\gamma}$, then the limiting phase will take only two values ${\alpha}$ and ${\beta}$ or ${\beta}$ and ${\gamma}$, depending on the constraint ${\int_\Omega u=c}$. This means that functionals of the form we presented above cannot be represented as a ${\Gamma}$-limit when the function ${W}$ is scalar, but with more than two zeros. This obstacle can be overcome by passing to the multidimensional case. This approach is presented by Sisto Baldo in [1] and we will present the ideas of this approach below.

## Master 5

March 1, 2013 1 comment

(If you are interested check out: Parts 1, 2, 3, 4, and the Shape Optimization page)

The notion of ${\Gamma}$-convergence was introduced by E. De Giorgi and T. Franzioni in [1]. For an introduction in the subject see the books [2], [3], by Andrea Braides and the free document [4] by the same author.

Definition 1 Let ${X}$ be a metric space, and for ${\varepsilon >0}$ let be given ${F_\varepsilon : X \rightarrow [0,\infty]}$. We say that ${F_\varepsilon}$ ${\Gamma}$-converges to ${F}$ on ${X}$ as ${\varepsilon \rightarrow 0}$, and we write ${\Gamma-\lim F_\varepsilon =F}$ or ${F_\varepsilon \stackrel{\Gamma}{\longrightarrow} F}$, if the following conditions hold:

• (LI) For every ${u \in X}$ and every sequence ${(u_\varepsilon)}$ such that ${u_\varepsilon \rightarrow u}$ in ${X}$ we have

$\displaystyle \liminf_{\varepsilon \rightarrow 0}F_\varepsilon(u_\varepsilon)\geq F(u)$

• (LS) For every ${u \in X}$ there exists a sequence ${(u_\varepsilon)}$ such that ${u_\varepsilon \rightarrow u}$ in ${X}$ and

$\displaystyle \limsup_{\varepsilon \rightarrow 0}F_\varepsilon(u_\varepsilon)\leq F(u).$

## Modica-Mortola Theorem

The notion of ${\Gamma}$-convergence was introduced by E. De Giorgi and T. Franzioni in the article Su un tipo di convergenza variazionale 1975.

Let ${X}$ be a metric space, and for ${\varepsilon >0}$ let be given ${F_\varepsilon : X \rightarrow [0,\infty]}$. We say that ${F_\varepsilon}$ ${\Gamma}$-converges to ${F}$ on ${X}$ as ${\varepsilon \rightarrow 0}$, and we write ${\Gamma-\lim F_\varepsilon =F}$, if the following conditions hold:

• (LI) For every ${u \in X}$ and every sequence ${(u_\varepsilon)}$ such that ${u_\varepsilon \rightarrow u}$ in ${X}$ we have

$\displaystyle \liminf_{\varepsilon \rightarrow 0}F_\varepsilon(u_\varepsilon)\geq F(u)$

• (LS) For every ${u \in X}$ there exists a sequence ${(u_\varepsilon)}$ such that ${u_\varepsilon \rightarrow u}$ in ${X}$ and

$\displaystyle \limsup_{\varepsilon \rightarrow 0}F_\varepsilon(u_\varepsilon)\leq F(u).$

Categories: shape optimization