We study a Hopf algebroid, ℋ, naturally associated to the groupoid Uδn ⋉ Un. We show that classes in the Hopf cyclic cohomology of ℋ can be used to define secondary characteristic classes of trivialized flat Un-bundles. For example, there is a cyclic class which corresponds to the universal transgressed Chern character and which gives rise to the continuous part of the ρ-invariant of Atiyah–Patodi–Singer. Moreover, these cyclic classes are shown to extend to pair with the K-theory of the associated C*-algebra. This point of view gives leads to homotopy invariance results for certain characteristic numbers. In particular, we define a subgroup of the cohomology of a group analogous to the Gelfand–Fuchs classes described by Connes  and show that the higher signatures associated to them are homotopy invariant.
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Jerome Kaminker, Xiang Tang, Hopf algebroids and secondary characteristic classes. J. Noncommut. Geom. 3 (2009), no. 1, pp. 1–25DOI 10.4171/JNCG/28