A Strengthened Alexandrov Maximum Principle or Uniform Hölder Continuity for Solutions of the Monge--Ampère Equation with Bounded Right-Hand Side

This article is about the convex solution $u$ of the Monge–Ampère equation on an at least 2-dimensional open bounded convex domain with Dirichlet boundary data and nonnegative bounded right-hand side. For convex functions with zero boundary data, an Alexandrov maximum principle $\mathrm{\mid }u(x)\mathrm{\mid }\le C\mathrm{dist}(x,\mathrm{\partial }\mathrm{\Omega }{)}^{\alpha }$ is equivalent to (uniform) Hölder continuity with the same constant and exponent. Convex $\alpha$-Hölder continuous functions are $W^{1,p}$ for $p<1\mathrm{/}(1-\alpha )$. We prove Hölder continuity with the exponent $\alpha =2\mathrm{/}n$ for $n\ge 3$ and any $\alpha \in (0,1)$ for $n=2$, provided that the boundary data satisfy this Hölder continuity, and show that these bounds for the exponent are sharp. The only means is to bound the Hessian determinant of a certain explicit function on an $n$-dimensional cylinder and to use the comparison princple.