The Imprimitivity Fell Bundle

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.