Coorbit theory is a powerful machinery that constructs a family of Banach spaces, the socalled coorbit spaces, from well-behaved unitary representations of locally compact groups. A core feature of coorbit spaces is that they can be discretized in a way that reflects the geometry of the underlying locally compact group. Many established function spaces such as modulation spaces, Besov spaces, Sobolev-Shubin spaces, and shearlet spaces are examples of coorbit spaces.The goal of this survey is to give an overview of coorbit theory with the aim of presenting the main ideas in an accessible manner. Coorbit theory is generally seen as a complicated theory, filled with both technicalities and conceptual difficulties. Faced with this obstacle, we feel obliged to convince the reader of the theory's elegance. As such, this survey is a showcase of coorbit theory and should be treated as a stepping stone to more complete sources. in Section 4.3. We end in Section 4.4 by giving references to recent developments related to embeddings between coorbit spaces and generalizations of coorbit theory.Acknowledgments: I have received advice and concrete suggestions from many researchers throughout the writing process. I would in particular like to thank Stine Marie Berge, Franz Luef, and Felix Voigtlaender for illuminating discussions and helpful comments. Finally, I would like to express my gratitude to everyone who participated in the seminar course I gave on coorbit theory at the Norwegian University of Science and Technology during the fall of 2020.
Starting OutWe start by giving an overview of preliminary topics, namely locally compact groups, unitary representations, and basic properties of the (generalized) wavelet transform. Most of this material is fairly standard, and is mainly collected from the books [34,42,16,36,21]. We aim for a suitable generality and present concrete examples as we go along.
Prelude on Locally Compact GroupsThe first order of business is to get acquainted with locally compact groups.Definition 2.1. A locally compact group is a locally compact Hausdorff topological space G that is simultaneously a group such that the multiplication and inversion mapsFor us, the object of main importance on a locally compact group is the left Haar measure: Recall that a Radon measure is Borel measure that is finite on compact sets, inner regular on open sets, and outer regular on all Borel sets. Do not worry if you are rusty on the measure-theoretic nonsense; we will never use these technical conditions explicitly. The important point is that each locally compact group G can be equipped with a unique (up to a positive constant) left-invariant Radon measure µ L , that is, µ L satisfies µ L (xE) = µ L (E) for all x ∈ G and every Borel set E ⊂ G. We call the measure µ L the left Haar measure of the group G. The existence of the left Haar measure implies that any locally compact group is canonically equipped with a measure-theoretic setting.As the terminology indicates, there is also a right Haar measure µ R on any locally ...