The Imprimitivity Fell Bundle
Anna Duwenig
TL;DR
The paper develops a Fell-bundle analogue of imprimitivity by introducing demi-equivalences and associating to them an imprimitivity Fell bundle $\mathbb{K}_{\mathscr{B}}(\mathscr{M})=\mathscr{M}\otimes_{\mathscr{B}}\mathscr{M}^{op}$ over the imprimitivity groupoid $\mathcal{G}=X\times_{\mathcal{H}}X^{op}$. It proves that this bundle is (up to isomorphism) the unique Fell bundle equivalent to $\mathscr{B}$ via $\mathscr{M}$, and, when $\mathscr{M}$ also furnishes an $(\mathscr{A},\mathscr{B})$-equivalence, yields a canonical isomorphism $\mathbb{K}(\mathscr{M}_{\mathscr{B}}) \cong \mathscr{A}$. This generalizes the classical $A$-compact operators picture, recovers Kumjian's stabilization trick, and unifies various imprimitivity results (including extensions of Green’s theorem and fixed-point constructions) within the Fell-bundle framework. The approach provides a versatile Morita-equivalence toolkit for groupoid-based C*-algebras and their bundles, with potential applications to KK-theory and noncommutative geometry.
Abstract
Given a full right-Hilbert C*-module $\mathbf{X}$ over a C*-algebra $A$, the set $\mathbb{K}_{A}(\mathbf{X})$ of $A$-compact operators on $\mathbf{X}$ is the (up to isomorphism) unique C*-algebra that is strongly Morita equivalent to the coefficient algebra $A$ via $\mathbf{X}$. As bimodule, $\mathbb{K}_{A}(\mathbf{X})$ can also be thought of as the balanced tensor product $\mathbf{X}\otimes_{A} \mathbf{X}^{\mathrm{op}}$, and so the latter naturally becomes a C*-algebra. We generalize both of these facts to the world of Fell bundles over groupoids: Suppose $\mathscr{B}$ is a Fell bundle over a groupoid $\mathcal{H}$ and $\mathscr{M}$ an upper semi-continuous Banach bundle over a principal right $\mathcal{H}$-space $X$. If $\mathscr{M}$ carries a right-action of $\mathscr{B}$ and a sufficiently nice $\mathscr{B}$-valued inner product, then its imprimitivity Fell bundle $\mathbb{K}_{\mathscr{B}}(\mathscr{M})=\mathscr{M}\otimes_{\mathscr{B}} \mathscr{M}^{\mathrm{op}}$ is a Fell bundle over the imprimitivity groupoid of $X$, and it is the unique Fell bundle that is equivalent to $\mathscr{B}$ via $\mathscr{M}$. We show that $\mathbb{K}_{\mathscr{B}}(\mathscr{M})$ generalizes the 'higher order' compact operators of Abadie and Ferraro in the case of saturated bundles over groups, and that the theorem recovers results such as Kumjian's Stabilization trick.