Singular traces, dimensions, and Novikov-Shubin invariants

Abstract

In Alain Connes noncommutative geometry, the question of the existence of a non-trivial integral can be described in terms of the singular traceability of the compact operator |D|^(-d), D being the Dirac operator, namely of the existence of a finite non-trivial singular trace on the ideal generated by |D|^(-d). A condition on the non triviality of the Dixmier logarithmic trace on the above mentioned ideal has been given by Connes in terms of the cohomological dimension of the Chern character of the phase of D. In this paper we show that, under suitable regularity conditions on the eigenvalue sequence of |D|, the dimension d can be uniquely determined by imposing that |D|^(-d) is singularly traceable, thus providing a geometric measure theoretic definition for d. More precisely, defining d as the inverse of the polynomial order of |D|^(-1), the operator |D|^(-d) always produces a singular trace which gives rise to a non-trivial integration procedure, even when the logarithmic trace fails. In the second part of the paper we discuss large scale counterparts of the geometric measure theoretic dimension for the case of covering manifolds. We prove that in the case of the coverings studied by Atiyah, the inverse of the polynomial order at 0 of Delta_p^(-1/2), defined via the Atiyah trace, coincides with the Novikov-Shubin number alpha_p. As a consequence, Novikov- Shubin numbers can be considered as (asymptotic) dimensions in the sense of geometric measure theory, since the corresponding power of Delta_p^(-1/2) produces a singular trace. Generalizations of the mentioned results on covering manifolds to the case of open manifolds with bounded geometry are studied in math/9809040