Aleksandrov-Bakelman-Pucci estimates

The celebrated "Alexandroff-Bakelman-Pucci Maximum Principle (often abbreviated often as "ABP Estimate") is a pointwise estimate for weak solutions of elliptic equations. It is a fundamental result which is the backbone of the regularity theory of fully nonlinear second order elliptic equations (reference Caffarelli-Cabré) and more recently for Fully nonlinear integro-differential equations (reference Caffarelli-Silvestre).

The classical Alexsandroff-Bakelman-Pucci Theorem

Let $u$ be a viscosity supersolution of the linear equation:

\[ a_ij(x) u_ij(x) \leq f(x) \;\; x \in B_1\] \[ u \leq 0 \;\; x \in \partial B_1\]

where the coefficients $a_ij(x)$ are only assumed to be measurable functions such that for positive constants $\lambda<\Lambda$ we have

\[ \lambda |\xi|^2 \leq a_ij(x) \xi_i\xi_j \leq \Lambda |\xi|^2 \;\;\forall \xi \in \mathbb{R}^n \]

Then,

\[ \sup \limits_{B_1}\; u^n \leq C_{n,\lambda,\Lambda} \int_{\{ u=\Gamma_u \} } f_+^n dx \]