Short Communication: On multiple roots in Descartes' Rule and their distance to roots of higher derivatives

  • Authors:
  • Arno Eigenwillig

  • Affiliations:
  • Max-Planck-Institut für Informatik, 66123 Saarbrücken, Germany

  • Venue:
  • Journal of Computational and Applied Mathematics
  • Year:
  • 2007

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Abstract

If an open interval I contains a k-fold root @a of a real polynomial f, then, after transforming I to (0,~), Descartes' Rule of Signs counts exactly k roots of f in I, provided I is such that Descartes' Rule counts no roots of the kth derivative of f. We give a simple proof using the Bernstein basis. The above condition on I holds if its width does not exceed the minimum distance @s from @a to any complex root of the kth derivative. We relate @s to the minimum distance s from @a to any other complex root of f using Szego's composition theorem. For integer polynomials, log(1/@s) obeys the same asymptotic worst-case bound as log(1/s).