On the maximum value of Jacobi polynomials

  • Authors:
  • Ilia Krasikov

  • Affiliations:
  • Department of Mathematical Sciences, Brunel University Uxbridge, UK

  • Venue:
  • Journal of Approximation Theory
  • Year:
  • 2005

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Abstract

A remarkable inequalily, with utterly explicit constants, established by Erdélyi, Magnus, and Nevai, states that for x ≥ β -1/2, the orthonormal Jacobi polynomials Pk(x,β)(x) satisfy max|x|≤1 {(1 - x)α+1/2(1+x)β+1/2(Pk(α,β)(x))2} = O(α) [Erdélyi et al., Generalized Jacobi weights, Christoffel functions, and Jacobi polynomials, SIAM J. Math. Anal. 25 (1994), 602 614]. They conjectured that the real order of the maximum is O(x1/2). Here we will make half a way towards this conjecture by proving a new inequality which improves their result by a factor of order {1/x + 1/k)-1/3. We also confirm the conjecture, even in a stronger form, in some limiting cases.