Weighted maximal regularity estimates and solvability of non-smooth elliptic systems I

TL;DR

This paper introduces new methods for solving divergence form elliptic systems with non-smooth coefficients near Lipschitz boundaries, establishing boundary estimates, solution characterizations, and well-posedness results.

Contribution

It develops a novel approach to analyze elliptic systems with coefficients close to those independent of the transversal variable, extending maximal regularity estimates and boundary value problem solvability.

Findings

Established boundary behavior and a priori estimates for elliptic systems.

Proved well-posedness of Dirichlet, Neumann, and regularity problems under small Carleson norm perturbations.

Extended maximal regularity estimates to systems in higher dimensions and non-symmetric cases.

Abstract

We develop new solvability methods for divergence form second order, real and complex, elliptic systems above Lipschitz graphs, with $L_2$ boundary data. The coefficients $A$ may depend on all variables, but are assumed to be close to coefficients $A_0$ that are independent of the coordinate transversal to the boundary, in the Carleson sense $\|A-A_0\|_C$ defined by Dahlberg. We obtain a number of {\em a priori} estimates and boundary behaviour results under finiteness of $\|A-A_0\|_C$. Our methods yield full characterization of weak solutions, whose gradients have $L_2$ estimates of a non-tangential maximal function or of the square function, via an integral representation acting on the conormal gradient, with a singular operator-valued kernel. Also, the non-tangential maximal function of a weak solution is controlled in $L_2$ by the square function of its gradient. This estimate is new for systems in such generality, and even for real non-symmetric equations in dimension 3 or higher. The existence of a proof {\em a priori} to well-posedness, is also a new fact. As corollaries, we obtain well-posedness of the Dirichlet, Neumann and Dirichlet regularity problems under smallness of $\|A-A_0\|_C$ and well-posedness for $A_0$, improving earlier results for real symmetric equations. Our methods build on an algebraic reduction to a first order system first made for coefficients $A_0$ by the two authors and A. McIntosh in order to use functional calculus related to the Kato conjecture solution, and the main analytic tool for coefficients $A$ is an operational calculus to prove weighted maximal regularity estimates.