A new characterization of Triebel-Lizorkin spaces on $\mathbb R^n$

Abstract

In this paper, the authors characterize the Triebel-Lizorkin space $\dot F^\alpha_{p,q}(\mathbb{R}^n)$ via a new square function

$$S_{\alpha,q}(f)(x)=\left\{\sum_{k\in\mathbb{Z}} 2^{k\alpha q}\left|\frac1{|B(x,2^{-k})|}\int_{B(x,2^{-k})}[f(x)-f(y)]\,dy \right|^q \right\}^{1/q}$$

where $f\in L^1_{\operatorname{loc}}({\mathbb R}^n)\cap \mathcal{S}'({\mathbb R}^n)$, $x\in{\mathbb R}^n$, $\alpha\in(0,2)$ and $p, q\in(1,\infty]$. Similar characterizations are also established for Triebel-Lizorkin spaces $\dot F^\alpha_{p,q}(\mathbb{R}^n)$ with $\alpha\in(0,\infty)\setminus 2{\mathbb N}$ and $p,q\in(1,\,\infty]$, and for Besov spaces $\dot B^\alpha_{p,q}(\mathbb{R}^n)$ with $\alpha\in(0,\infty)\setminus 2{\mathbb N}$, $p\in(1,\infty]$ and $q\in(0,\infty]$.