Poisson-Lie T-duality as a boundary phenomenon of Chern-Simons theory

We give a "holographic" explanation of Poisson-Lie T-duality in terms of Chern-Simons theory (or, more generally, in terms of Courant sigma-models) with appropriate boundary conditions.Comment: 17 pages (a mistake in the energy-momentum tensor on p.2 corrected

The geometry of the maximum likelihood of Cauchy-like distributions

A simple way of obtaining robust estimates of the "center" (or the "location") and of the "scatter" of a dataset is to use the maximum likelihood estimate with a class of heavy-tailed distributions, regardless of the "true" distribution generating the data. We observe that the maximum likelihood problem for the Cauchy distributions, which have particularly heavy tails, is geodesically convex and therefore efficiently solvable (Cauchy distributions are parametrized by the upper half plane, i.e. by the hyperbolic plane). Moreover, it has an appealing geometrical meaning: the datapoints, living on the boundary of the hyperbolic plane, are attracting the parameter by unit forces, and we search the point where these forces are in equilibrium. This picture generalizes to several classes of multivariate distributions with heavy tails, including, in particular, the multivariate Cauchy distributions. The hyperbolic plane gets replaced by symmetric spaces of noncompact type. Geodesic convexity gives us an efficient numerical solution of the maximum likelihood problem for these distribution classes. This can then be used for robust estimates of location and spread, thanks to the heavy tails of these distributions.Comment: 17 pages; v3: graphs in the Appendix (p.14) corrected; v4: important reference [Fl\"uge-Ruh] adde

On Deformation Quantization of Poisson-Lie Groups and Moduli Spaces of Flat Connections

We give simple explicit formulas for deformation quantization of Poisson-Lie groups and of similar Poisson manifolds which can be represented as moduli spaces of flat connections on surfaces. The star products depend on a choice of Drinfe\v{l}d associator and are obtained by applying certain monoidal functors (fusion and reduction) to commutative algebras in Drinfe\v{l}d categories. From a geometric point of view this construction can be understood as a quantization of the quasi-Poisson structures on moduli spaces of flat connections.Comment: 11 page