Quantitative John-Nirenberg inequalities at different scales

TL;DR

This paper develops a general abstract framework to derive quantitative John-Nirenberg inequalities at various scales, extending classical results to more general measures and function spaces, including Orlicz and variable exponent spaces.

Contribution

It introduces a unified abstract estimate that recovers sharp inequalities and extends them to new settings like doubling measures and advanced function spaces.

Findings

Derived a general estimate encompassing classical John-Nirenberg inequalities.

Extended inequalities to Orlicz and variable exponent spaces.

Provided a new characterization of Muckenhoupt's A_infinity weights.

Abstract

We provide an abstract estimate of the form \[ \|f-f_{Q,\mu}\|_{X \left(Q,\frac{\mathrm{d} \mu}{Y(Q)}\right)}\leq c(\mu,Y)\psi(X)\|f\|_{\mathrm{BMO}(\mathrm{d}\mu)} \] for all cubes $Q$ in $\mathbb{R}^n$ and every function $f\in \mathrm{BMO}(\mathrm{d}\mu)$, where $\mu$ is a doubling measure in $\mathbb{R}^n$, $Y$ is some positive functional defined on cubes, $\|\cdot \|_{X \left(Q,\frac{\mathrm{d} w}{w(Q)}\right)}$ is a sufficiently good quasi-norm and $c(\mu,Y)$ and $\psi(X)$ are positive constants depending on $\mu$ and $Y$, and $X$, respectively. That abstract scheme allows us to recover the sharp estimate \[ \|f-f_{Q,\mu}\|_{L^p \left(Q,\frac{\mathrm{d} \mu(x)}{\mu(Q)}\right)}\leq c(\mu)p\|f\|_{\mathrm{BMO}(\mathrm{d}\mu)}, \qquad p\geq1 \] for every cube $Q$ and every $f\in \mathrm{BMO}(\mathrm{d}\mu)$, which is known to be equivalent to the John-Nirenberg inequality, and also enables us to obtain quantitative counterparts when $L^p$ is replaced by suitable strong and weak Orlicz spaces and $L^{p(\cdot)}$ spaces. Besides the aforementioned results we also generalize Theorem 1.2 in [OPRRR20] to the setting of doubling measures and obtain a new characterization of Muckenhoupt's $A_\infty$ weights.