$Local^{3}$ Index Theorem

TL;DR

This paper extends the local index theorem by introducing a localized cyclic homology framework, enabling a genuine Chern character for a non-idempotent operator associated with elliptic pseudo-differential operators.

Contribution

It proposes a novel approach to the local index theorem by localizing cyclic homology and defining a genuine Chern character for a non-idempotent operator.

Findings

Operators satisfy a modified idempotent-like identity

Genuine Chern character constructed in localized cyclic homology

New framework for local index theorem with support filtering

Abstract

$Local^{3}$ Index Theorem means $Local(Local(Local \;Index \; Theorem)))$. $Local \; Index \; Theorem$ is the Connes-Moscovici local index theorem \cite{Connes-Moscovici1}, \cite{Connes-Moscovici2}. The second "Local" refers to the cyclic homology localised to a certain separable subring of the ground algebra, while the last one refers to Alexander-Spanier type cyclic homology. The Connes-Moscovici work is based on the operator $R(A) = \mathbf{P} - \mathbf{e}$ associated to the elliptic pseudo-differential operator $A$ on the smooth manifold $M$, where $\mathbf{P}$, $\mathbf{e}$ are idempotents, see \cite{Connes-Moscovici1}, Pg. 353. The operator $R(A)$ has two main merits: it is a smoothing operator and its distributional kernel is situated in an arbitrarily small neighbourhood of the diagonal in $M \times M$. The operator $R(A)$ has also two setbacks: -i) it is not an idempotent (and therefore it does not have a genuine Connes-Chern character); -ii) even if it were an idempotent, its Connes-Chern character would belong to the cyclic homology of the algebra of smoothing operators (with \emph{arbitrary} supports, which is \emph{trivial}. This paper presents a new solution to the difficulties raised by the two setbacks. For which concerns -i), we show that although $R(A)$ is not an idempotent, it satisfies the identity $ (\mathbf{R}(A))^{2} \;=\; \mathbf{R}(A) - [\mathbf{R}(A) . e + e .\mathbf{R}(A) ]. $ We show that the operator $R(A)$ has a genuine Chern character provided the cyclic homology complex of the algebra of smoothing operators is \emph{localised} to the separable sub-algebra $\Lambda = \mathbb{C} + \mathbb{C} . e$, see Sect. 8.1. For which concerns -ii), we introduce the notion of \emph{local} cyclic homology; this is constructed on the foot-steps of the Alexander-Spanier homology, i.e. by filtering the chains of the cyclic homology complex of the algebra of smoothing operators by their distributional support, see Sect. 7. Using these new instruments, we give a reformulation of the Connes-Moscovici local Index Theorem, see Theorem 23, Sect. 9. As a corollary of this theorem, we show that the \emph{local} cyclic homology of the algebra of smoothing operators is at least as big as the Alexander-Spanier homology of the base manifold. The present reformulation of Connes-Moscovici local index theorem opens the way to new investigations, see Sect. 10.