Sobolev orthogonal polynomials: balance and asymptotics

M. Alfaro, J. J. Moreno-Balcazar, A. Pena, M. L. Rezola

Published 2006-06-23, updated 2007-05-02Version 2

Let $\mu_0$ and $\mu_1$ be measures supported on an unbounded interval and $S_{n,\lambda_n}$ the extremal varying Sobolev polynomial which minimizes \begin{equation*} < P, P >_{\lambda_n}=\int P^2 d\mu_0 + \lambda_n \int P'^2 d\mu_1, \quad \lambda_n >0 \end{equation*} \noindent in the class of all monic polynomials of degree $n$. The goal of this paper is twofold. On one hand, we discuss how to balance both terms of this inner product, that is, how to choose a sequence $(\lambda_n)$ such that both measures $\mu_0$ and $\mu_1$ play a role in the asymptotics of $(S_{n, \lambda_n}).$ On the other, we apply such ideas to the case when both $\mu_0$ and $\mu_1$ are Freud weights. Asymptotics for the corresponding $S_{n, \lambda_n}$ are computed, illustrating the accuracy of the choice of $\lambda_n .$