A generalized Pell’s equation for a class of multivariate orthogonal polynomials

Lasserre, Jean; Xu, Yuan

doi:10.1090/tran/9200

A generalized Pell’s equation for a class of multivariate orthogonal polynomials
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by Jean B. Lasserre and Yuan Xu;

Trans. Amer. Math. Soc.

DOI: https://doi.org/10.1090/tran/9200

Published electronically: May 8, 2025

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Abstract:

We extend the polynomial Pell’s equation satisfied by univariate Chebyshev polynomials on $[-1,1]$ from one variable to several variables, using orthogonal polynomials on regular domains that include cubes, balls, and simplexes of arbitrary dimension. Moreover, we show that such an equation is strongly connected (i) to a certificate of positivity (from real algebraic geometry) on the domain, as well as (ii) to the Christoffel functions of the equilibrium measure on the domain. In addition, the solution to Pell’s equation reflects an extremal property of orthonormal polynomials associated with an entropy-like criterion.

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A generalized Pell’s equation for a class of multivariate orthogonal polynomials
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Abstract: