The inner kernel theorem for a certain Segal algebra

The Segal algebra ${\textbf{S}}_{0}(G)$ is well defined for arbitrary locally compact Abelian Hausdorff (LCA) groups $G$. Despite the fact that it is a Banach space it is possible to derive a kernel theorem similar to the Schwartz kernel theorem, of course without making use of the Schwartz kernel theorem. First we characterize the bounded linear operators from ${\textbf{S}}_{0}(G_1)$ to ${\textbf{S}}_{0}'(G_2)$ by distributions in ${\textbf{S}}_{0}'(G_1 \times G_2)$. We call this the "outer kernel theorem". The "inner kernel theorem" is concerned with the characterization of those linear operators which have kernels in the subspace ${\textbf{S}}_{0}(G_1 \times G_2)$, the main subject of this manuscript. We provide a description of such operators as regularizing operators in our context, mapping ${\textbf{S}}_{0}'(G_1)$ into test functions in ${\textbf{S}}_{0}(G_2)$, in a $w^{*}$-to norm continuous manner. The presentation provides a detailed functional analytic treatment of the situation and applies to the case of general LCA groups, without recurrence to the use of so-called Wilson bases, which have been used for the case of elementary LCA groups. The approach is then used in order to describe natural laws of composition which imitate the composition of linear mappings via matrix multiplications, now in a continuous setting. We use here that in a suitable (weak) form these operators approximate general operators. We also provide an explanation and mathematical justification used by engineers explaining in which sense pure frequencies "integrate" to a Dirac delta distribution

Compactness Criteria in Function Spaces

The classical criterion for compactness in Banach spaces of functions can be reformulated into a simple tightness condition in the time-frequency domain. This description preserves more explicitly the symmetry between time and frequency than the classical conditions. The result is first stated and proved for L^2(R^d), and then generalized to coorbit spaces. As special cases, we obtain new characterizations of compactness in Besov-Triebel-Lizorkin spaces, modulation spaces and Bargmann-Fock spaces

Flexible Gabor-wavelet atomic decompositions for L2-Sobolev spaces

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