1. Estimates for the Largest Critical Value of Tn(k).
- Author
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Naidenov, Nikola and Nikolov, Geno
- Subjects
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JACOBI polynomials , *CHEBYSHEV polynomials , *HYPERGEOMETRIC functions , *ABSOLUTE value , *BESSEL functions , *GAUSSIAN quadrature formulas - Abstract
For T n (x) = cos n arccos x , x ∈ [ - 1 , 1 ] , the n-th Chebyshev polynomial of the first kind, we study the quantity τ n , k : = | T n (k) (ω n , k) | T n (k) (1) , 1 ≤ k ≤ n - 2 , where T n (k) is the k-th derivative of T n and ω n , k is the largest zero of T n (k + 1) . Since the absolute values of the local extrema of T n (k) increase monotonically towards the end-points of [ - 1 , 1 ] , the value τ n , k shows how small is the largest critical value of T n (k) relative to its global maximum T n (k) (1) . This is a continuation of our (joint with Alexei Shadrin) paper "On the largest critical value of T n (k) ", SIAM J. Math. Anal.50(3), 2018, 2389–2408, where upper bounds and asymptotic formuae for τ n , k have been obtained on the basis of the Schaeffer–Duffin pointwise upper bound for polynomials with absolute value not exceeding 1 in [ - 1 , 1 ] . We exploit a 1996 result of Knut Petras about the weights of the Gaussian quadrature formulae associated with the ultraspherical weight function w λ (x) = (1 - x 2) λ - 1 / 2 to find an explicit (modulo ω n , k ) formula for τ n , k 2 . This enables us to prove a lower bound and to refine the previously obtained upper bounds for τ n , k . The explicit formula admits also a new derivation of the asymptotic formula approximating τ n , k for n → ∞ . The new approach is simpler, without using deep results about the ordinates of the Bessel function, and allows to better analyze the sharpness of the estimates. [ABSTRACT FROM AUTHOR]
- Published
- 2024
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