Sharp estimates for the commutators of the Hilbert, Riesz transforms and the Beurling-Ahlfors operator on weighted Lebesgue spaces

TL;DR

This paper establishes sharp quadratic bounds for the operator norms of commutators of Hilbert, Riesz, and Beurling transforms with BMO functions on weighted Lebesgue spaces, extending known linear bounds.

Contribution

It introduces a general theorem for Haar shift operators to derive sharp quadratic bounds for these commutators on weighted spaces.

Findings

Operator norms depend quadratically on the A2-characteristic of the weight.

The results are sharp for 1 < p < ∞.

The approach uses Bellman function methods and a general Haar shift theorem.

Abstract

We prove that the operator norm on weighted Lebesgue space L2(w) of the commutators of the Hilbert, Riesz and Beurling transforms with a BMO function b depends quadratically on the A2-characteristic of the weight, as opposed to the linear dependence known to hold for the operators themselves. It is known that the operator norms of these commutators can be controlled by the norm of the commutator with appropriate Haar shift operators, and we prove the estimate for these commutators. For the shift operator corresponding to the Hilbert transform we use Bellman function methods, however there is now a general theorem for a class of Haar shift operators that can be used instead to deduce similar results. We invoke this general theorem to obtain the corresponding result for the Riesz transforms and the Beurling-Ahlfors operator. We can then extrapolate to Lp(w), and the results are sharp for 1 < p < 1.