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Initial approximations in Durand-Kerner’s root finding method. (English) Zbl 0628.65038

The author provides a procedure for calculating the initial radius \(r_ 0\) to be used in obtaining the roots of \(p(z)=a_ 0z^ n+...+a_{n- 1}z+a_ n\), \(a_ 0a_ n\neq 0\) by using Durand-Kerner’s method: \(z_ k^{(0)}=r_ 0\exp (2\pi ki/n),\) \(k=0,1,...,n\) \[ z_ k^{(j+1)}=z_ k^{(j)}-p(z_ k^{(j)})/\{a_ 0\prod_{i<k}(z_ k^{(j)}-z_ i^{(j+1)})\prod_{i>k}(z_ k^{(j)}-z_ i^{(j)})\}. \] The procedure is: i) for \(i=1,...,n\) if \(a_ i\neq 0\quad u_ i=2| a_ i/a_ 0|^{1/i}\), for \(j=0,...,n-1\) if \(a_ j\neq 0\quad V_ j=1/(2| a_ j/a_ n|^{1/(n-j)})\); ii) discard \(u_ i\) and \(v_ j\) for which \(u_ i-\bar u\), and \(\bar v-v_ j\) are maximal. Recalculate \(\bar u\) and \(\bar v.\) iii) \(r_ 0=(\bar u+\bar v)/2\). The author gives four examples illustrating that his initial radius \(r_ 0\) gives an efficient procedure in terms of number of evaluations of p(z) and that his strategy is comparable to that of L. W. Ehrlich’s using \(\min \{| z| | p(z)=0\}<r_ 0<\max \{| z| | p(z)=0\}.\)
Reviewer: I.Imam

MSC:

65H05 Numerical computation of solutions to single equations
65E05 General theory of numerical methods in complex analysis (potential theory, etc.)
30C15 Zeros of polynomials, rational functions, and other analytic functions of one complex variable (e.g., zeros of functions with bounded Dirichlet integral)
Full Text: DOI

References:

[1] L. W. Ehrlich,A modified Newton method for polynomials. Comm. ACM 10 (1967), 107–108. · Zbl 0148.39004 · doi:10.1145/363067.363115
[2] Göran Kjellberg,Two observations on Durand-Kerner’s root finding method. BIT 24 (1984), 556–559. · Zbl 0551.65025 · doi:10.1007/BF01934913
[3] M. A. Jenkins and J. F. Traub,A three-stage algorithm for real polynomials, SIAM J. Numer. Anal. 7 (1970), 545–566. · Zbl 0237.65034 · doi:10.1137/0707045
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