Optimal exponents in weighted estimates without examples

TL;DR

This paper introduces a general method to determine the optimal exponents in weighted inequalities for various operators, linking their bounds to unweighted norms and classical theorems, without needing specific examples.

Contribution

The authors develop a unified approach to establish sharp exponents in weighted estimates for a broad class of operators, including maximal, Calderón-Zygmund, and fractional operators.

Findings

Derived lower bounds for exponents in weighted inequalities.

Proved sharpness of exponents for Bochner-Riesz multipliers.

Extended results to maximal operators over general bases.

Abstract

We present a general approach for proving the optimality of the exponents on weighted estimates. We show that if an operator $T$ satisfies a bound like $$ \|T\|_{L^{p}(w)}\le c\, [w]^{\beta}_{A_p} \qquad w \in A_{p}, $$ then the optimal lower bound for $\beta$ is closely related to the asymptotic behaviour of the unweighted $L^p$ norm $\|T\|_{L^p(\mathbb{R}^n)}$ as $p$ goes to 1 and $+\infty$, which is related to Yano's classical extrapolation theorem. By combining these results with the known weighted inequalities, we derive the sharpness of the exponents, without building any specific example, for a wide class of operators including maximal-type, Calder\'on--Zygmund and fractional operators. In particular, we obtain a lower bound for the best possible exponent for Bochner-Riesz multipliers. We also present a new result concerning a continuum family of maximal operators on the scale of logarithmic Orlicz functions. Further, our method allows to consider in a unified way maximal operators defined over very general Muckenhoupt bases.