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Projectively invariant Hilbert-Schmidt kernels and convolution type operators. (English) Zbl 1272.46045

Summary: We consider positive definite kernels which are invariant under a multiplier and an action of a semigroup with involution, and construct the associated projective isometric representation on a Hilbert \(C^*\)-module. We introduce the notion of \(C^*\)-valued Hilbert-Schmidt kernels associated with two sequences and construct the corresponding reproducing Hilbert \(C^*\)-module. We also discuss projective invariance of Hilbert-Schmidt kernels. We prove that the range of a convolution type operator associated with a Hilbert-Schmidt kernel coincides with the reproducing Hilbert \(C^*\)-module associated with its convolution kernel. We show that the integral operator associated with a Hilbert-Schmidt kernel is Hilbert-Schmidt. Finally, we discuss a relation between an integral type operator and a convolution type operator.

MSC:

46L08 \(C^*\)-modules
46E22 Hilbert spaces with reproducing kernels (= (proper) functional Hilbert spaces, including de Branges-Rovnyak and other structured spaces)
46C50 Generalizations of inner products (semi-inner products, partial inner products, etc.)
46N55 Applications of functional analysis in statistical physics
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