# Journal of Noncommutative Geometry

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**Volume 10, Issue 1, 2016, pp. 1–28**

**DOI: 10.4171/JNCG/227**

Published online: 2016-03-22

Projective families of Dirac operators on a Banach Lie groupoid

Pedram Hekmati and Jouko Mickelsson^{[1]}(1) University of Helsinki, Finland

We introduce a Banach Lie group $G$ of unitary operators subject to a natural trace condition. We compute the homotopy groups of $G$, describe its cohomology and construct an $S^1$-central extension. We show that the central extension determines a non-trivial gerbe on the action Lie groupoid $G \ltimes \frak k$, where $\frak k$ denotes the Hilbert space of self-adjoint Hilbertâ€“Schmidt operators. With an eye towards constructing elements in twisted K-theory, we prove the existence of a cubic Dirac operator $\mathbb D$ in a suitable completion of the quantum Weil algebra $\mathcal{U}(\frak{g}) \otimes Cl(\frak{k})$, which is subsequently extended to a projective family of self-adjoint operators $\mathbb D_A$ on $G \ltimes \frak k$. While the kernel of $\mathbb D_A$ is infinite-dimensional, we show that there is still a notion of finite reducibility at every point, which suggests a generalized definition of twisted K-theory for action Lie groupoids.

*Keywords: *Twisted K-theory, Dirac operators, Banach Lie groups

Hekmati Pedram, Mickelsson Jouko: Projective families of Dirac operators on a Banach Lie groupoid. *J. Noncommut. Geom.* 10 (2016), 1-28. doi: 10.4171/JNCG/227