Nonlinear steepest descent approach to orthogonality on elliptic curves

We consider the recently introduced notion of denominators of Padé--like approximation problems on a Riemann surface. These denominators are related as in the classical case to the notion of orthogonality over a contour. We investigate a specific setup where the Riemann surface is a real elliptic curve and the measure of orthogonality is supported on one of the two real ovals. Using a characterization in terms of a Riemann--Hilbert problem, we determine the strong asymptotic behaviour of the corresponding orthogonal functions for large degree. The theory of vector bundles and the non-abelian Cauchy kernel play a prominent role even in this simplified setting, indicating the new challenges that the steepest descent method on a Riemann surface has to overcome.

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