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Quadratic differentials and asymptotics of Laguerre polynomials with varying complex parameters. (English) Zbl 1295.30015
Summary: We study the asymptotics (as $$n\to\infty$$) of sequences of Laguerre polynomials with varying complex parameters $$\alpha$$ depending on the degree $$n$$. More precisely, we assume that $$\alpha_n=nA_n$$ and $$\lim_nA_n=A\in\mathbb C$$. This study has been carried out previously only for $$\alpha_n\in\mathbb R$$, but complex values of $$A$$ introduce an asymmetry that makes the problem more difficult. The main ingredient of the asymptotic analysis is the right choice of the contour of orthogonality, which requires the analysis of the global structure of trajectories of an associated quadratic differential on the complex plane, which may have an independent interest. While the weak asymptotics is obtained by reduction to the theorem of Gonchar-Rakhmanov-Stahl, the strong asymptotic results are derived via the non-commutative steepest descent analysis based on the Riemann-Hilbert characterization of the Laguerre polynomials.

##### MSC:
 30C10 Polynomials and rational functions of one complex variable 33C45 Orthogonal polynomials and functions of hypergeometric type (Jacobi, Laguerre, Hermite, Askey scheme, etc.) 30E15 Asymptotic representations in the complex plane
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