Nijenhuis operators on homogeneous spaces related to $C^*$-algebras

TL;DR

This paper investigates Nijenhuis operators on homogeneous spaces derived from unital non-simple $C^*$-algebras, exploring their properties, conditions for Nijenhuis structures, and applications to specific algebra classes.

Contribution

It introduces a framework for analyzing vector bundle maps as Nijenhuis operators on homogeneous spaces related to $C^*$-algebras, including conditions and examples.

Findings

Characterization of when vector bundle maps are Nijenhuis operators

Conditions for almost complex structures on $G/K$

Examples involving Toeplitz algebra and crossed products

Abstract

For a unital non-simple $C^*$-algebra $\mathcal A$ we consider its Banach--Lie group $G$ of invertible elements. For a given closed ideal $\mathfrak k$ in $\mathcal A$, we consider the embedded Banach--Lie subgroup $K$ of $G$ of elements differing from the unit element by an element in $\mathfrak k$. We study vector bundle maps of the tangent space of the homogeneous space $G/K$, induced by an admissible bounded operator on $\mathcal A$. In particular, we discuss when this vector bundle map is a Nijenhuis operator in $G/K$. The special case of almost complex structures in $G/K$ is also addressed. Examples for particular classes of $C^*$-algebras are presented, including the Toeplitz algebra and crossed products by $\mathbb Z$.