Alexandrov's estimate revisited

Abstract

Alexandrov's estimate states that if $\Omega$ is a bounded open convex domain in ${\mathbb R}^n$ and $u:\bar \Omega\to {\mathbb R}$ is a convex solution of the Monge-Ampere equation $\det D^2 u = f$ that vanishes on $\partial \Omega$, then \[ |u(x) - u(y)| \le \omega(|x-y|)(\int_\Omega f)^{1/n} \qquad \mbox{for }\omega(\delta) = C_n\,\mbox{diam}(\Omega)^{\frac{n-1}n} \delta^{1/n}. \] We establish a variety of improvements of this, depending on the geometry of $\partial \Omega$. For example, we show that if the curvature is bounded away from $0$, then the estimate remains valid if $\omega(\delta)$ is replaced by $C_\Omega \delta^{\frac 12 + \frac 1{2n}}$. We determine the sharp constant $C_\Omega$ when $n=2$, and when $n\ge 3$ and $\partial \Omega$ is $C^2$, we determine the sharp asymptotics of the optimal modulus of continuity $\omega_\Omega(\delta)$ as $\delta\to 0$. For arbitrary convex domains, we characterize the scaling of the optimal modulus $\omega_\Omega$. Under very mild nondegeneracy conditions, our results yield the improved Holder estimate, $\omega_\Omega(\delta) \le C \delta^\alpha$ for some $\alpha>1/n$.