INTRODUCTION 3
an element of the cyclic cohomology of A, computes the index pairing defined
by a Fredholm module. Any cocycle in the same cohomology class as the Chern
character will therefore also compute the index pairing. In this memoir we define
several cocycles that represent the Chern character and which are expressed in
terms of the unbounded operator D. These cocycles generalise those found in
[15–17] (where semifinite versions of the local index formula were first proved) to
the nonunital case. We have to prove that these additional cocycles, including the
residue cocycle, are in the class of the Chern character in the (b, B)-complex.
Our main result (stated in Theorem 3.33 of Chapter 3) is then an expression for
the index pairing using a nonunital version of the semifinite local index formula of
[15,16], which is in turn a generalisation to the setting of semifinite von Neumann
algebras of the original Connes-Moscovici [25] formula. Our noncommutative index
formula is given by a sum of residues of zeta functions and is easily recognisable
as a direct generalisation of the unital formulas of [15,16,25]. We emphasise that
even for the standard B(H) case our local index formula is new.
One of the main difficulties that we have to overcome is that while there is a well
understood theory of Fredholm (or Kasparov) modules for nonunital algebras, the
‘right framework’ for working with unbounded representatives of these K-homology
classes has proved elusive. We believe that we have found the appropriate formalism
and the resulting residue index formula provides evidence that the approach to
spectral triples over nonunital algebras initiated in [10] is fundamentally sound
and leads to interesting applications. Related ideas on the K-homology point of
view for relative index theorems are to be found in [52], [9] and [19], and further
references in these texts.
We also discuss some fully noncommutative applications in Chapter 5, including
the type I spectral triple of the Moyal plane constructed in [27] and semifinite spec-
tral triples arising from torus actions on
C∗-algebras,
but leave other applications,
such as those to the results in [44], [46] and [60], to elsewhere.
To explain how we arrived at the technical framework described here, consider
the simplest possible classical case, where H = L2(R), D =
d
idx
and A is a certain
∗-subalgebra of the algebra of smooth functions on R. Let P = χ[0,∞)(D) be the
projection defined using the functional calculus and the characteristic function of
the half-line and let u be a unitary in A such that u 1 converges to zero at ±∞
‘sufficiently rapidly’. Then the classical Gohberg-Krein theory gives a formula for
the index of the Fredholm operator PMuP where Mu is the operator of multiplica-
tion by u on L2(R). In proving this theorem for general symbols u, one confronts
the classical question (studied in depth in [56]) of when an operator of the form
(Mu 1)(1 + D2)−s/2, s 0, is trace class. In the general noncommutative setting
of this article, this question and generalisations must still be confronted and this is
done in Chapter 1.
The results for manifolds. In the case of closed manifolds, the local index
formula in noncommutative geometry (due to Connes-Moscovici [25]) can serve as a
starting point to derive the Atiyah-Singer index theorem for Dirac type operators.
This proceeds by a Getzler type argument enunciated in this setting by Ponge,
[47], though similar arguments have been used previously with the JLO cocycle
as a starting point in [7, 23]. While there is already a version of this Connes-
Moscovici formula that applies in the noncompact case [50], it relies heavily on the
use of compact support assumptions.
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