Compactness Characterizations of Commutators on Ball Banach Function Spaces

Abstract: Let $X$ be a ball Banach function space on ${\mathbb R}^n$. Let $\Omega$ be a Lipschitz function on the unit sphere of ${\mathbb R}^n$,which is homogeneous of degree zero and has mean value zero, and let $T_\Omega$ be the convolutional singular integral operator with kernel $\Omega(\cdot)/|\cdot|^n$. In this article, under the assumption that the Hardy--Littlewood maximal operator $\mathcal{M}$ is bounded on both $X$ and its associated space, the authors prove that the commutator $[b,T_\Omega]$ is compact on $X$ if and only if $b\in{\rm CMO}({\mathbb R}^n)$. To achieve this, the authors mainly employ three key tools: some elaborate estimates, given in this article, on the norm in $X$ of the commutators and the characteristic functions of some measurable subset,which are implied by the assumed boundedness of ${\mathcal M}$ on $X$ and its associated space as well as the geometry of $\mathbb R^n$; the complete John--Nirenberg inequality in $X$ obtained by Y. Sawano et al.; the generalized Fr\'{e}chet--Kolmogorov theorem on $X$ also established in this article. All these results have a wide range of applications. Particularly, even when $X:=L^{p(\cdot)}({\mathbb R}^n)$ (the variable Lebesgue space), $X:=L^{\vec{p}}({\mathbb R}^n)$ (the mixed-norm Lebesgue space), $X:=L^\Phi({\mathbb R}^n)$ (the Orlicz space), and $X:=(E_\Phi^q)_t({\mathbb R}^n)$ (the Orlicz-slice space or the generalized amalgam space), all these results are new.