$\mathfrak{K}$-families and CPD-H-extendable families

TL;DR

This paper introduces the concept of $rak{K}$-families between Hilbert $C^*$-modules, providing a factorization theorem, characterizations, and extension properties related to CPD-kernels and dynamical systems.

Contribution

It defines $rak{K}$-families for Hilbert $C^*$-modules, establishes their factorization, and explores their extension and covariance properties in relation to CPD-kernels and crossed products.

Findings

$rak{K}$-families can be characterized via linking algebras.

Covariant $rak{K}$-families extend to crossed products.

Dilation theory relates to $rak{K}$-families and CPD-kernels.

Abstract

We introduce, for any set $S$, the concept of $\mathfrak{K}$-family between two Hilbert $C^*$-modules over two $C^*$-algebras, for a given completely positive definite (CPD-) kernel $\mathfrak{K}$ over $S$ between those $C^*$-algebras and obtain a factorization theorem for such $\mathfrak{K}$-families. If $\mathfrak{K}$ is a CPD-kernel and $E$ is a full Hilbert $C^*$-module, then any $\mathfrak{K}$-family which is covariant with respect to a dynamical system $(G,\eta,E)$ on $E$, extends to a $\tilde{\mathfrak{K}}$-family on the crossed product $E \times_\eta G$, where $\tilde{\mathfrak{K}}$ is a CPD-kernel. Several characterizations of $\mathfrak{K}$-families, under the assumption that ${E}$ is full, are obtained and covariant versions of these results are also given. One of these characterizations says that such $\mathfrak{K}$-families extend as CPD-kernels, between associated (extended) linking algebras, whose $(2,2)$-corner is a homomorphism and vice versa. We discuss a dilation theory of CPD-kernels in relation to $\mathfrak{K}$-families.