On the classification and modular extendability of E$_0$-semigroups on factors

TL;DR

This paper investigates the properties of E$_0$-semigroups on factors, focusing on modular extendability, classification by type, and the existence of non-extendable examples across various factor types.

Contribution

It establishes that modular extendability is invariant under cocycle conjugacy and tensoring, classifies E$_0$-semigroups by type, and constructs infinitely many non-extendable examples.

Findings

Modular extendability is a cocycle conjugacy invariant.

All types of modularly extendable E$_0$-semigroups exist on properly infinite factors.

Existence of infinitely many non-cocycle conjugate, non-extendable E$_0$-semigroups on hyperfinite factors.

Abstract

In this paper we study modular extendability and equimodularity of endomorphisms and E$_0$-semigroups on factors with respect to f.n.s. weights. We show that modular extendability is a property that does not depend on the choice of weights, it is a cocycle conjugacy invariant and it is preserved under tensoring. We say that a modularly extendable E$_0$-semigroup is of type EI, EII or EIII if its modular extension is of type I, II or III, respectively. We prove that all types exist on properly infinite factors. We also compute the coupling index and the relative commutant index for the CAR flows and $q$-CCR flows. As an application, by considering repeated tensors of the CAR flows we show that there are infinitely many non cocycle conjugate non-extendable $E_0$-semigroups on the hyperfinite factors of types II$_1$, II$_{\infty}$ and III$_\lambda$, for $\lambda \in (0,1)$.