$\alpha$-modulation spaces for step two stratified Lie groups

TL;DR

This paper introduces and studies $oldsymbol{ extalpha}$-modulation spaces on step two stratified Lie groups, extending Euclidean spaces to non-commutative settings and exploring their properties and embeddings.

Contribution

It defines $oldsymbol{ extalpha}$-modulation spaces on step two stratified Lie groups, analyzing their structure, non-triviality, and embedding properties, extending classical Euclidean function spaces.

Findings

Spaces are non-trivial and distinct from Euclidean counterparts.

Boundary cases $oldsymbol{ extalpha=0,1}$ exhibit non-standard symmetries.

Existence of geometric embeddings from Euclidean modulation spaces into group-based spaces.

Abstract

We define and investigate $\alpha$-modulation spaces $M_{p,q}^{s,\alpha}(G)$ associated to a step two stratified Lie group $G$ with rational structure constants. This is an extension of the Euclidean $\alpha$-modulation spaces $M_{p,q}^{s,\alpha}(\mathbb{R}^n)$ that act as intermediate spaces between the modulation spaces ($\alpha = 0$) in time-frequency analysis and the Besov spaces ($\alpha = 1$) in harmonic analysis. We will illustrate that the the group structure and dilation structure on $G$ affect the boundary cases $\alpha = 0,1$ where the spaces $M_{p,q}^{s}(G)$ and $\mathcal{B}_{p,q}^{s}(G)$ have non-standard translation and dilation symmetries. Moreover, we show that the spaces $M_{p,q}^{s,\alpha}(G)$ are non-trivial and generally distinct from their Euclidean counterparts. Finally, we examine how the metric geometry of the coverings $\mathcal{Q}(G)$ underlying the $\alpha = 0$ case $M_{p,q}^{s}(G)$ allows for the existence of geometric embeddings \[F:M_{p,q}^{s}(\mathbb{R}^k) \longrightarrow{} M_{p,q}^{s}(G),\] as long as $k$ (that only depends on $G$) is small enough. Our approach naturally gives rise to several open problems that is further elaborated at the end of the paper.