Muckenhoupt Weights Meet Brezis--Seeger--Van Schaftingen--Yung Formulae in Ball Banach Function Spaces

Abstract: In this article, via first establishing a weighted variant of the profound and far-reaching inequality obtained by A. Cohen, W. Dahmen, I. Daubechies, and R. DeVore in 2003, the authors give two new characterizations of Muckenhoupt weights. As an application, the authors further establish a representation formula of gradients with sharp parameters in ball Banach function spaces, which extends the famous formula obtained by H. Brezis, A. Seeger, J. Van Schaftingen, and P.-L. Yung in 2021 from classical Sobolev spaces to various different Sobolev-type spaces and gives an affirmative answer to the question in page 29 of [Calc. Var. Partial Differential Equations 62 (2023), Paper No. 234]. The most novelty of this article exists in subtly revealing the mutual equivalences among the Muckenhoupt weight, the weighted variant of the inequality of Cohen et al., and the weighted upper estimate of the formula of Brezis et al.