Matrix-Weighted Besov-Type and Triebel--Lizorkin-Type Spaces I: $A_p$-Dimensions of Matrix Weights and $\varphi$-Transform Characterizations

Abstract: Let $s\in{\mathbb R}$, $q\in (0,\infty]$, and $\tau\in[0,\infty)$. It is well known that Besov-type spaces $\dot B^{s,\tau}_{p,q}$ with $p\in (0,\infty]$ and Triebel--Lizorkin-type spaces $\dot F^{s,\tau}_{p,q}$ with $p\in (0,\infty)$ when $\tau\in [0,\infty)$ or with $p\in (0,\infty]$ when $\tau=0$ on $\mathbb{R}^n$ consist of a general family of function spaces that cover not only the well-known Besov and Triebel--Lizorkin spaces $\dot B^{s}_{p,q}$ and $\dot F^{s}_{p,q}$ (when $\tau=0$) but also several other function spaces of interest, such as Morrey spaces and $Q$ spaces. In three successive articles, the authors develop a complete real-variable theory of matrix-weighted Besov-type spaces $\dot B^{s,\tau}_{p,q}(W)$ and matrix-weighted Triebel--Lizorkin-type spaces $\dot F^{s,\tau}_{p,q}(W)$ on $\mathbb{R}^n$, where $W$ is a matrix-valued Muckenhoupt $A_p$ weight. This article is the first one, whose main novelty exists in that the authors introduce the new concept, $A_p$-dimensions of matrix weights, and intensively study their properties, especially those elaborate properties expressed via reducing operators. The authors then introduce the spaces $\dot B^{s,\tau}_{p,q}(W)$ and $\dot F^{s,\tau}_{p,q}(W)$ and, using $A_p$-dimensions and their nice properties, the authors establish the $\varphi$-transform characterization of $\dot B^{s,\tau}_{p,q}(W)$ and $\dot F^{s,\tau}_{p,q}(W)$. The $A_p$-dimensions of matrix weights and their properties also enable the authors to obtain the sharp boundedness of almost diagonal operators on related sequence spaces in the subsequent second article and the optimal characterizations of molecules and wavelets, trace theorems, and the optimal boundedness of pseudo-differential operators and Calder\'on--Zygmund operators in the subsequent third article.