Wiener amalgams and product-convolution operatorsHans G. Feichtinger (NuHAG, Faculty Mathematics, University Vienna) given at GF2020, Ghent (03.09.20) id: 3684 length: min status: type: LINK-Presentation: https://nuhagphp.univie.ac.at/dateien/talks/3684_FeiGFS2020.pdf ABSTRACT: {\it Wiener amalgam spaces} (originally called {\it Wiener-type spaces} have been introduced in full generality in the summer of 1980. They allow to describe the {\it global behaviour} of some {\it local norm}. The most simple cases are spaces of the form $W(L^p,\ell^q)(R^d)$, with $1 \leq p,q \leq \infty$, where the $\ell^q$-sum of the local $L^p$-norms over the unit cubes sitting at $k \in Z^d$ is finite (and defines the norm). In the more general case one has to make use of somewhat smooth and uniform partitions of unity, such as the basis functions (shifted B-splines) for the space of cubic splines. {\it Product-convolution} or {convolution-product} operators are concatenations of pointwise multiplication and convolution operators. While pointwise multiplication may increase the decay of a given function or distribution it is clear that a convolution operator will typically improve the local properties (without changing the global behaviour). Moreover, such operators are good regularizers and thus appear in the theory of (ultra-)distributions, showing how to approximate distributions by test functions. We will discuss a few of such situations, and if time permits we will also indicate some new results (joint work with Stevan Pilipovic and Bojan Prangovski) concerning the characterization of some new translation and modulation invariant Banach spaces of functions. |