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## Re: [Help-gsl] (in)accuracy of gsl_poly_complex_solve for repeated roots

**From**: |
Brian Gough |

**Subject**: |
Re: [Help-gsl] (in)accuracy of gsl_poly_complex_solve for repeated roots? |

**Date**: |
Fri, 10 Jun 2005 19:23:48 +0100 |

Steven G. Johnson writes:
>* Hi, I noticed that gsl_poly_complex_solve seems to be surprisingly *
>* inaccurate. For example, if you ask it for the roots of 1 + 4x + 6x^2 + *
>* 4x^3 + x^4, which should have x = -1 as a four-fold root (note that the *
>* coefficients and solutions are exactly representable), it gives roots:*
Thanks, I will add a note about it in the manual. Higher multiplicity
roots are always more sensitive to numerical error as there is a
factor of (macheps)^(1/n) in the error for a root of multiplicity n.
--
Brian Gough