The unbounded Kasparov product by a differentiable module

TL;DR

This paper extends the unbounded Kasparov product to a broader class of modules by introducing a twisted setting that relaxes smoothness and projectivity conditions, enabling more flexible constructions in noncommutative geometry.

Contribution

It develops a new framework for the unbounded Kasparov product using twisted modules without smooth projectivity, broadening the applicability of unbounded KK-theory.

Findings

The unbounded Kasparov product can be defined in a twisted setting with weak assumptions.

The product recovers the classical Kasparov product after bounded transformation.

The approach bypasses limitations of existing recognition theorems like Kucerovsky's.

Abstract

In this paper we investigate the unbounded Kasparov product between a differentiable module and an unbounded cycle of a very general kind that includes all unbounded Kasparov modules and hence also all spectral triples. Our assumptions on the differentiable module are weak and we do in particular not require that it satisfies any kind of smooth projectivity conditions. The algebras that we work with are furthermore not required to possess a smooth approximate identity. The lack of an adequate projectivity condition on our differentiable module entails that the usual class of unbounded Kasparov modules is not flexible enough to accommodate the unbounded Kasparov product and it becomes necessary to twist the commutator condition by an automorphism. We show that the unbounded Kasparov product makes sense in this twisted setting and that it recovers the usual interior Kasparov product after taking bounded transforms. Since our unbounded cycles are twisted (or modular) we are not able to apply the work of Kucerovsky for recognizing unbounded representatives for the bounded Kasparov product, instead we rely directly on the connection criterion developed by Connes and Skandalis. In fact, since we do not impose any twisted Lipschitz regularity conditions on our unbounded cycles, even the passage from an unbounded cycle to a bounded Kasparov module requires a substantial amount of extra care.