Abstract
We show that for a uniformly elliptic divergence form operator L, defined in an open set \(\Omega \) with Ahlfors–David regular boundary, BMO solvability implies scale-invariant quantitative absolute continuity (the weak-\(A_\infty \) property) of elliptic-harmonic measure with respect to surface measure on \(\partial \Omega \). We do not impose any connectivity hypothesis, qualitative, or quantitative; in particular, we do not assume the Harnack Chain condition, even within individual connected components of \(\Omega \). In this generality, our results are new even for the Laplacian. Moreover, we obtain a partial converse, assuming in addition that \(\Omega \) satisfies an interior Corkscrew condition, in the special case that L is the Laplacian.
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Notes
It might be more accurate to refer to this property as “VMO-solvability,” but BMO solvability seems to be the established terminology in the literature. Under less austere circumstances, e.g., in a Lipschitz or (more generally) a Chord-arc domain, it can be seen that the two notions are ultimately equivalent; see [6] for a discussion of this point, although for the reader’s convenience, we shall show below (see Remark 4.5) that in fact the equivalence holds for a domain with an ADR boundary, in the presence of “\(S<N\) estimates” in \(L^p\) (thus, in particular, in the special case of the Laplacian, in a domain satisfying an interior Corkscrew condition). In the more general setting of our Theorem 1.1 this matter is not settled.
i.e., either \(\Omega \) is a bounded domain, or both \(\Omega \) and \(\partial \Omega \) are unbounded. We discuss a variant of our results, valid in the case that \(\partial \Omega \) is bounded but \(\Omega \) is unbounded, in Sect. 5.
The BMO estimate for f in (3.6) follows from the fact that \(\mathcal {M}(1_A)^{1/2}\) is an A\(_1\) weight with A\(_1\) constant depending only on dimension, and that the log of an A\(_1\) weight w belongs to BMO, with BMO norm depending only on the A\(_1\) constant of w; see, e.g., [8, Ch. 2, Theorems 3.3 and 3.4].
Recall that our ambient dimension is \(n+1\), with \(n\ge 2\), so that the fundamental solution of L decays to 0 at infinity.
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Acknowledgements
We are grateful to Simon Bortz for a suggestion which has simplified one of our arguments in Sect. 4. We also thank the referee for a careful reading of the manuscript, for suggesting numerous improvements to the exposition, and especially for pointing out an error in the original proof of Lemma 3.3. Finally, we thank Chema Martell for suggesting the approach that we have used in Sect. 5 to treat the case of an unbounded domain with bounded boundary. The authors were supported by NSF Grant Number DMS-1361701. Steve Hofmann was also supported by NSF Grant Number DMS-1664047
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Hofmann, S., Le, P. BMO Solvability and Absolute Continuity of Harmonic Measure. J Geom Anal 28, 3278–3299 (2018). https://doi.org/10.1007/s12220-017-9959-0
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DOI: https://doi.org/10.1007/s12220-017-9959-0
Keywords
- BMO
- Dirichlet problem
- Harmonic measure
- Divergence form elliptic equations
- Weak-\(A_\infty \)
- Ahlfors–David regularity
- Uniform rectifiability