Rates of convergence in W²_p-norm for the Monge-Amp\`ere equation

We develop discrete $W^2_p$-norm error estimates for the Oliker-Prussner method applied to the Monge-Amp\`ere equation. This is obtained by extending discrete Alexandroff estimates and showing that the contact set of a nodal function contains information on its second order difference. In addition, we show that the size of the complement of the contact set is controlled by the consistency of the method. Combining both observations, we show that the error estimate $\|u - u_h\|_{W^2_p} \leq C h^{1/p}$ if $p > d$ and $\|u - u_h\|_{W^2_p} \leq C h^{1/d} \big(\ln\left(\frac 1 h \right)\big)^{1/d} $ if $p \leq d$. Here the constant $C$ depends on $\|{u}\|_{C^{3,1}(\bar\Omega)}$, the dimension $d$, and the constant $p$. Numerical examples are given in two space dimensions and confirm that the estimate is sharp in several cases.