The nearest complex polynomial with a zero in a given complex domain

  • Authors:
  • Zhongxuan Luo;Wenyu Hu;Dongwoo Sheen

  • Affiliations:
  • School of Mathematical Sciences, Dalian University of Technology, Dalian 116024, China and School of Software, Dalian University of Technology, Dalian 116620, China;School of Mathematical Sciences, Dalian University of Technology, Dalian 116024, China;Department of Mathematics and Interdisciplinary Program in Computational Science & Technology, Seoul National University, Seoul 151-747, Republic of Korea

  • Venue:
  • Theoretical Computer Science
  • Year:
  • 2011

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Abstract

Given a univariate complex polynomial f and a closed complex domain D, whose boundary C is a curve parameterized by a piecewise rational function, we propose two computational algorithms for finding a univariate complex polynomial f@? such that f@? has a zero in D and the distance between f and f@? is minimal. Our approach is composed of two steps. First, in the case of D consisting of one point @a, we give explicit formulas of f@? and the minimal distance in terms of @a. Next, the case of a general closed domain D is considered by using the property that a nearest polynomial f@? has a zero on the boundary C. The curve C is parameterized piecewisely, and on each piece we search for the minimum of the distance between f and f@?. At this step we exploit the explicit formula of the minimal distance as a function of a point @a. Then the global minimum and the nearest polynomial are obtained by comparing the piecewise minima. Some examples are presented: one of them confirms that the distance between a nearest complex polynomial and a given polynomial is less than that between a nearest real polynomial and the given polynomial.