### Solving the Monge-Ampere equation (continued… and finished).

I have been postponing this post for over a week due to lack of time, but finally here it is. This post ought to finish a series of past posts (here and here) where I have been describing the proof of existence of classical solutions to the Dirichlet problem for the Monge-Ampere equationvia the continuity method. Via the method of continuity  we reduced the question of existence of classical solutions to the problem of proving good a priori estimates for classical solutions, namely, we were trying to prove

Theorem (A priori estimate for the Monge-Ampere equation) Let $u$ be a smooth solution of

$det(D^2u)=\psi \mbox{ in } B_1$

$u= 0 \mbox{ on } \partial B_1$

There is a constant $C>0$ (depending only on $n$ and the $C^3$ norm of $\psi$) such that

$||u||_{C^{2,\alpha}}\leq C$

Last time we got almost there, using the maximum principle and the right barriers we proved the estimate

$||u||_{C^{1,1}(B_1)}\leq C$

Which is still not strong enough for our needs, so in order to finish the proof of the a priori estimate, we are going to use a powerful interior estimate for concave elliptic equations, proved independently by L.C. Evans and N. Krylov in the 80’s:
Theorem (Evans-Krylov) Let $u$ be a $C^{1,1}$ solution of the elliptic equation

$F(D^2u)=f(x)$ for $x \in B_1$

if $F$ is concave (or convex), then we have the following interior estimate

$||u||_{C^{2,\alpha_0}(B_{\frac{1}{2}})}\leq C||u||_{C^{1,1}(B_1)}$

where $C$ depends only on $F$, and $\alpha_0$ is a universal constant.

This a well known theorem, a couple of places where one can read it are the book of Gilbarg and Trudinger (last edition), or the book of Caffarelli and Cabre. More recently, Caffarelli and Silvestre have come up with a shorter proof, still based on the original ideas of Evans and Krylov, this proof is available in arxiv. Maybe I will talk about the proof in some other post, but for now I am just going to quote the result.

The Evans-Krylov theorem is an interior result, we need also control at the boundary, that is provided by a result of Krylov

Theorem (Krylov) Let $u$ be a solution to our equation, there is a universal $\beta$ and constant $C$ controlled by $|u|_\infty$ such that  for $x \in B_1$ and $y \in \partial B_1$ we have

$|D^2u(x)-D^2u(y)|\leq C|x-y|^\beta$

This actually a corollary of Krylov’s theorem, which is a more general and remarkable result about equations in non-divergence form with measurable coefficients, but again I want to focus on the Monge-Ampere equation, I will talk about Krylov’s theorem some other time, a good place to read about it is the last chapter of Kazdan’s book. With these two tools its a standard argument to show that for a constant $C$ controlled by the previous two, and for $\alpha=min(\alpha_0,\beta)$, we have

$|D^2u(x)-D^2u(y)|\leq C|x-y|^\alpha$

I won’t do it in detail, but the proof is not too hard: basically, if the two points are closer to each other than to the boundary, then the Evans-Krylov estimate (properly scaled) gives us the inequality above, otherwise, the two points are closer to the boundary than to each other, so by the estimate of Krylov we get the same inequality in this case, and thats it!

…and that finishes the proof!. I did not present the most general result to simplify the presentation(at least for the weaker a priori estimates, which is where I did most of the details), but one can work in a more general domain (as long as it is convex) and have arbitrary boundary conditions. A much more general result, which includes not only the Monge-Ampere equation but also the $k-$Hessian equation, was proven  in a  paper by Caffarelli, Nirenberg and Spruck.