In relatively nice geometric settings, in particular, on Lipschitz domains,
absolute continuity of elliptic measure with respect to the surface measure is
equivalent to Carleson measure estimates, to square function estimates, and to
-approximability,
for solutions to the second-order divergence-form elliptic partial differential equations
.
In more general situations, notably, in an open set
with a
uniformly rectifiable boundary, absolute continuity of elliptic measure with respect to
the surface measure may fail, already for the Laplacian. In the present paper, extending
and clarifying our previous work (Duke Math J. 165:12 (2016), 2331–2389), we
demonstrate that nonetheless, Carleson measure estimates, square function estimates, and
-approximability
remain valid in such
,
for solutions of
,
provided that such solutions enjoy these properties in Lipschitz subdomains of
.
Moreover, we establish a general real-variable transference principle, from
Lipschitz to chord-arc domains, and from chord-arc to open sets with uniformly
rectifiable boundary, that is not restricted to harmonic functions or even to solutions
of elliptic equations. In particular, this allows one to deduce the first Carleson
measure estimates and square function bounds for higher-order systems on open sets
with uniformly rectifiable boundaries and to treat subsolutions and subharmonic
functions.