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We quantify the Sobolev space norm of the Beltrami resolvent \((I- \mu S)^{-1}\), where \(S\) is the Beurling–Ahlfors transform, in terms of the corresponding Sobolev space norm of the dilatation \(\mu\) in the critical and supercritical ranges. Our estimate entails as a consequence quantitative self-improvement inequalities of Caccioppoli type for quasiregular distributions with dilatations in \(W^{1,p}\), \(p \ge 2\). Our proof strategy is then adapted to yield quantitative estimates for the resolvent \((I-\mu S_\Omega)^{-1}\) of the Beltrami equation on a sufficiently regular domain \(\Omega\), with \(\mu\in W^{1,p}(\Omega)\). Here, \(S_\Omega\) is the compression of \(S\) to a domain \(\Omega\). Our proofs do not rely on the compactness or commutator arguments previously employed in related literature. Instead, they leverage the weighted Sobolev estimates for compressions of Calderón–Zygmund operators to domains, recently obtained by the authors, to extend the Astala–Iwaniec–Saksman technique to higher regularities. 

Abstract We prove that the class of trilinear multiplier forms with singularity over a one-dimensional subspace, including the bilinear Hilbert transform, admits bounded $L^p$-extension to triples of intermediate $$\operatorname{UMD}$$ spaces. No other assumption, for instance of Rademacher maximal function type, is made on the triple of $$\operatorname{UMD}$$ spaces. Among the novelties in our analysis is an extension of the phase-space projection technique to the $$\textrm{UMD}$$-valued setting. This is then employed to obtain appropriate single-tree estimates by appealing to the $$\textrm{UMD}$$-valued bound for bilinear Calderón–Zygmund operators recently obtained by the same authors. 

Abstract We establish the sharp growth rate, in terms of cardinality, of the $L^p$ norms of the maximal Hilbert transform $$H_\Omega $$ along finite subsets of a finite order lacunary set of directions $$\Omega \subset \mathbb{R}^3$$, answering a question of Parcet and Rogers in dimension $n=3$. Our result is the first sharp estimate for maximal directional singular integrals in dimensions greater than 2. The proof relies on a representation of the maximal directional Hilbert transform in terms of a model maximal operator associated to compositions of 2D angular multipliers, as well as on the usage of weighted norm inequalities, and their extrapolation, in the directional setting.