Strong stability for the Wulff inequality with a crystalline norm (1910.09515v1)

Abstract: Let $K$ be a convex polyhedron and $\mathscr F$ its Wulff energy, and let $\mathscr C(K)$ denote the set of convex polyhedra close to $K$ whose faces are parallel to those of $K$. We show that, for sufficiently small $\epsilon$, all $\epsilon$-minimizers belong to $\mathscr C(K)$. As a consequence of this result we obtain the following sharp stability inequality for crystalline norms: There exist $\gamma=\gamma(K,n)>0$ and $\sigma=\sigma(K,n)>0$ such that, whenever $|E|=|K|$ and $|E\Delta K|\le \sigma$ then $$ \mathscr F(E) - \mathscr F(K^a)\ge \gamma |E \Delta K^a| \qquad \text{for some }K^a \in \mathscr C(K). $$ In other words, the Wulff energy $\mathscr F$ grows very fast (with power $1$) away from the set $\mathscr C(K).$ The set $K^a \in \mathscr C(K)$ appearing in the formula above can be informally thought as a sort of "projection" of $E$ on the set $\mathscr C(K).$ Another corollary of our result is a very strong rigidity result for crystals: For crystalline surface tensions, minimizers of $\mathscr F(E)+\int_E g$ with small mass are polyhedra with sides parallel to the ones of $K$. In other words, for small mass, the potential energy cannot destroy the crystalline structure of minimizers. This extends to arbitrary dimensions a two-dimensional result obtained in [9].