we show that the logarithmic trace of Szegö projectors introduced by K. Hirachi [15] for CR structures and extended in [8] to contact structures vanishes identically.Keywords: CR manifolds, contact manifolds, Toeplitz operators, residual trace. MSC2000: 58J40, 32A25, 53D10, 53D55.In [15] K. Hirachi showed that the logarithmic trace of the Szegö projector is an invariant of the CR structure. In [8] I showed that it is also defined for generalized Szegö projectors associated to a contact structure (definitions recalled below, sect.4), that it is a contact invariant, and that it vanishes if the base manifold is a 3-sphere, with arbitrary contact structure (not necessarily the canonical one). Here we show that it always vanishes. For this use the fact that this logarithmic trace is the residual trace of the identity (definitions recalled below, sect.5), and show that it always vanishes, because the Toeplitz algebra associated to a contact structure can be embedded in the Toeplitz algebra of a sphere, where the identity maps of all 'good' Toeplitz modules have zero residual trace.
NotationsWe first recall the notions that we will use. Most of the material below in §1-5 is not new; we have just recalled briefly the definitions and useful properties, and send back to the literature for further details (cf. [16,17,21,18]).If X is a smooth manifold we denote T • X ⊂ T * X the set of non-zero covectors. A complex subspace Z corresponds to an ideal IZ ⊂ C ∞ (T • X, C)). Z is conic (homogeneous) if it is generated by homogeneous functions. It is smooth if IZ is locally generated by k = codim Z functions with linearly independent derivatives. If Z is smooth, it is involutive if IZ is stable by the Poisson bracket (in local coordinates {f, g} = ∂f ∂ξ j ∂g ∂x j − ∂f ∂x j ∂g ∂ξ j ); it is ≫ 0 if locally IZ has generators ui, vj (1 ≤ i ≤ p, 1 ≤ j ≤ q, p + q =codim Z) such that the vj are real, ui complex, and the matrix 1 i ({u k ,ū l ) is hermitian ≫ 0. The real part ZR is then a smooth real submanifold of T • X, whose ideal is generated by the Re ui, Im ui, vj. If I is ≫ 0, it is exactly determined by its formal germ (Taylor expansion) along the set of real points of ZR.A Fourier integral operator (FIO) from Y to X, is a linear operator from functions or distributions on Y to same on X defined as a locally finite sum of oscillating integralswhere φ is a phase function (homogeneous w.r. to θ), a a symbol function. Here we will only consider regular symbols, i.e. which are asymptotic sums a ∼ k≥0 a m−k where a m−k is homogeneous of degree m − k, k an integer. m could be any complex number. There is also a notion of vector FIO, acting on sections of vector bundles, which we will not use here.