The Monge-Ampere (whether real or complex, whether in a domain or on a manifold) equations usually studied are of the type $F(u, \nabla u, \mathrm{Hess}(u)) = 0$ where $F$ is a concave function of $\mathrm{Hess}(u)$. This is done for the purpose of $C^{2, \alpha}$ estimates. My question is: Has any work been done in the non-concave setting? (I know of Harvey and Lawson's work that gives us viscosity solutions, but I wish for more regular solutions) Especially with regard to the $C^{2,\alpha}$ estimate?



Monge Ampere equations (concavity) 
Do a google search for "Fully nonlinear uniformly elliptic equations" and you will turn up a lot of information. The best estimates for viscosity solutions are $C^{1,\alpha}$, in general, where $\alpha > 0$ is very tiny. This was work in the 1980s due to Caffarelli following the important work of Krylov-Safonov. There are very recent counterexamples due to Nadirashvili and Vladut which show that this is the best you can get. The latest and greatest (arXiv: 1111.0329) has counterexamples in dimension $N \geq 5$. 


If $F$ is convex or concave in the Hessian, then there are $C^{2,\alpha}$ estimates. This is the Evans-Krylov theorem. There are also estimates for certain classes of nonconvex equations with special structure (see the work of Cabre-Caffarelli). 


And (ahem) if you will permit me to advertise my own work with Luis Silvestre and Charlie Smart (arxiv:1103.3677), we have a partial regularity result for a general nonlinearity, stating that the singular set is small: for some $\alpha> 0$ (very tiny) the solutions are $C^{2,1-\alpha}$ off a set of Hausdorff dimension $N-\alpha$. 


