Robust approximate zeros

  • Authors:
  • Vikram Sharma;Zilin Du;Chee K. Yap

  • Affiliations:
  • Courant Institute of Mathematical Sciences, New York University, New York, NY;Courant Institute of Mathematical Sciences, New York University, New York, NY;Courant Institute of Mathematical Sciences, New York University, New York, NY

  • Venue:
  • ESA'05 Proceedings of the 13th annual European conference on Algorithms
  • Year:
  • 2005

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Abstract

Smale’s notion of an approximate zero of an analytic function f: C →C is extended to take into account the errors incurred in the evaluation of the Newton operator. Call this stronger notion a robust approximate zero.. We develop a corresponding robust point estimate for such zeros: we prove that if z0∈ C satisfies α(f,z0)z0 is a robust approximate zero, with the associated zero z* lying in the closed disc ${\overline B}(z_{0},\frac{0.07}{f,z_{0}}$. Here α(f,z), γ(f,z) are standard functions in point estimates. Suppose f(z) is an L-bit integer square-free polynomial of degree d. Using our new algorithm, we can compute an n-bit absolute approximation of z*∈IR starting from a bigfloat z0, in time O[dM(n+d2(L+lg d)lg(n+L))], where M(n) is the complexity of multiplying n-bit integers.