Modular curvature for noncommutative two-tori

Connes, Alain ¹   ; Moscovici, Henri ²

¹ Collége de France, 3, rue d’Ulm, Paris, F-75005 France – and – IHES, 91440 Bures-Sur-Yvette, France – and – The Ohio State University, Columbus, Ohio 43210
² Department of Mathematics, The Ohio State University, Columbus, Ohio 43210

Journal of the American Mathematical Society, Tome 27 (2014) no. 3, pp. 639-684

Résumé

In this paper we investigate the curvature of conformal deformations by noncommutative Weyl factors of a flat metric on a noncommutative 2-torus, by analyzing in the framework of spectral triples functionals associated to perturbed Dolbeault operators. The analogue of Gaussian curvature turns out to be a sum of two functions in the modular operator corresponding to the non-tracial weight defined by the conformal factor, applied to expressions involving the derivatives of the same factor. The first is a generating function for the Bernoulli numbers and is applied to the noncommutative Laplacian of the conformal factor, while the second is a two-variable function and is applied to a quadratic form in the first derivatives of the factor. Further outcomes of the paper include a variational proof of the Gauss-Bonnet theorem for noncommutative 2-tori, the modular analogue of Polyakov’s conformal anomaly formula for regularized determinants of Laplacians, a conceptual understanding of the modular curvature as gradient of the Ray-Singer analytic torsion, and the proof using operator positivity that the scale invariant version of the latter assumes its extreme value only at the flat metric.

Détail

DOI : 10.1090/S0894-0347-2014-00793-1

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