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Electronic Resource
Springer
Constructive approximation 8 (1992), S. 463-535
ISSN: 1432-0940
Keywords: Primary 41A17 ; 42C05 ; Secondary 41A10 ; Freud weights ; Exponential weights ; Orthonormal polynomials ; Christoffel functions ; Markov-Bernstein inequalities ; Potentials ; Discretization of potentials ; Nevai's conjecture
Source: Springer Online Journal Archives 1860-2000
Topics: Mathematics
Notes: Abstract We obtain upper and lower bounds for Christoffel functions for Freud weights by relatively new methods, including a new way to estimate discretization of potentials. We then deduce bounds for orthogonal polynomials onℝ thereby largely resolving a 1976 conjecture of P. Nevai. For example, let W:=e −Q, whereQ:ℝ→ℝ is even and continuous inℝ, Q" is continuous in (0, ∞) andQ '〉0 in (0, ∞), while, for someA, B, $$1〈 A \leqslant \frac{{(d/dx)(xQ'(x))}}{{Q'(x)}} \leqslant B,x \in (0,\infty )$$ Leta n denote thenth Mhaskar-Rahmanov-Saff number forQ, andL〉0. Then, uniformly forn≥1 and |x|≤a n (1+Ln −2/3), $$\lambda _n (W^2 ,x) \sim \frac{{a_n }}{n}W^2 (x)\left( {\max \left\{ {n^{ - 2/3} ,1 - \frac{{|x|}}{{a_n }}} \right\}} \right)^{ - 1/2}$$ Moreover, for all x εℝ, we can replace ∼ by ≥. In particular, these results apply toW(x):=exp(-|x|α), α〉1. We also obtain lower bounds for allx εℝ, when onlyA〉0, but this necessarily requires a more complicated formulation. We deduce that thenth orthonormal plynomialp n (W 2, ·). forW 2 satisfies $$\mathop {\sup }\limits_{x \in \mathbb{R}} |p_n (W^2 ,x)|W(x)\left| {1 - \frac{{|x|}}{{a_n }}} \right|^{1/4} \sim a_n^{ - 1/2}$$ and $$\mathop {\sup }\limits_{x \in \mathbb{R}} |p_n (W^2 ,x)|W(x) \sim a_n^{ - 1/2} n^{1/6} .$$ In particular, this applies toW(x):=exp(-|x|α), α〉1.
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