Modular curvature for noncommutative two-tori

Connes, Alain; Moscovici, Henri

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

Modular curvature for noncommutative two-tori
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by Alain Connes and Henri Moscovici

J. Amer. Math. Soc. 27 (2014), 639-684

DOI: https://doi.org/10.1090/S0894-0347-2014-00793-1

Published electronically: April 8, 2014

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Abstract:

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.

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