RAPHAËL PONGEAs a step towards proving an index theorem for hypoelliptic operators on Heisenberg manifolds, including for those on CR and contact manifolds, we construct an analogue for Heisenberg manifolds of Connes' tangent groupoid of a manifold. As is well known for a Heisenberg manifold (M, H) the relevant notion of tangent bundle is rather that of a Lie group bundle of graded 2-step nilpotent Lie groups G M. We define the tangent groupoid of (M, H) as a differentiable groupoid Ᏻ H M encoding the smooth deformation of M × M to G M. In particular, this construction makes a crucial use of a refined notion of privileged coordinates and of a tangent-approximation result for Heisenberg diffeomorphisms.
IntroductionA somewhat long standing open question is the existence of an index theorem for geometric operators on contact and CR manifolds. In this context the operators are not elliptic, so we cannot apply the classical index theorem of Atiyah-Singer [1968a;1968b]. The natural pseudodifferential tool to deal with hypoelliptic operators on contact and CR manifolds is provided by the Heisenberg calculus of BealsGreiner [1988] and Taylor [1984]. The latter holds in full generality for Heisenberg manifolds, that is, manifolds M together with a distinguished hyperplane bundle H ⊂ T M. This definition includes that of CR and contact manifolds, as well as that of codimension one foliations and confoliations. Therefore, what we would like to have is an analogue of the Atiyah-Singer theorem for hypoelliptic operators on Heisenberg manifolds.There are various proofs of the Atiyah-Singer index theorem. A simple and fairly general proof is that of Connes [1994, Sect. II.5]. A salient feature in Connes' proof is the use of the tangent groupoid of a manifold, that is, the differentiable groupoid encoding the smooth deformation of M × M to T M (see [Connes 1994;Hilsum and Skandalis 1987]).In this paper, as a step towards proving an index theorem for hypoelliptic operators on Heisenberg manifolds, we construct an analogue for Heisenberg manifolds MSC2000: primary 58H05; secondary 53C10, 53D10, 32V05.