Localisations of Half-Closed Modules and the Unbounded Kasparov Product
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Publication:6115157
DOI10.1093/IMRN/RNAC065zbMATH Open1520.19009arXiv2006.10616OpenAlexW3036227542MaRDI QIDQ6115157
Publication date: 12 July 2023
Published in: IMRN. International Mathematics Research Notices (Search for Journal in Brave)
Abstract: In the context of the Kasparov product in unbounded KK-theory, a well-known theorem by Kucerovsky provides sufficient conditions for an unbounded Kasparov module to represent the (internal) Kasparov product of two other unbounded Kasparov modules. In this article, we discuss several improved and generalised variants of Kucerovsky's theorem. First, we provide a generalisation which relaxes the positivity condition, by replacing the lower bound by a relative lower bound. Second, we also discuss Kucerovsky's theorem in the context of half-closed modules, which generalise unbounded Kasparov modules to symmetric (rather than self-adjoint) operators. In order to deal with the positivity condition for such non-self-adjoint operators, we introduce a fairly general localisation procedure, which (using a suitable approximate unit) provides a 'localised representative' for the KK-class of a half-closed module. Using this localisation procedure, we then prove several variants of Kucerovsky's theorem for half-closed modules. A distinct advantage of the localised approach, also in the special case of self-adjoint operators (i.e., for unbounded Kasparov modules), is that the (global) positivity condition in Kucerovsky's original theorem is replaced by a (less restrictive) 'local' positivity condition, which is closer in spirit to the well-known Connes-Skandalis theorem in the bounded picture of KK-theory.
Full work available at URL: https://arxiv.org/abs/2006.10616
Noncommutative differential geometry (46L87) Spin and Spin({}^c) geometry (53C27) Kasparov theory ((KK)-theory) (19K35) Noncommutative geometry (à la Connes) (58B34)
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