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## Re: [Help-gsl] gsl_poly_solve_cubic problem

**From**: |
Brian Gough |

**Subject**: |
Re: [Help-gsl] gsl_poly_solve_cubic problem |

**Date**: |
Sun, 06 Jan 2008 21:47:09 +0000 |

**User-agent**: |
Wanderlust/2.14.0 (Africa) Emacs/22.1 Mule/5.0 (SAKAKI) |

At Sat, 5 Jan 2008 11:31:40 -0500,
Andrew W. Steiner wrote:
>* *
>* I briefly took a look at this. The GSL cubic routine is failing*
>* because 'R' and 'Q'*
>* are not precisely zero. If the CR2==CQ3 test succeeded, the routine might*
>* also give the correct answer, except that Q is actually negative in this*
>* context, so that sqrt(Q) is not finite.*
Some of this comes from the fact that the decimal coefficients and
roots of (x-1.1)^3 do not have exact representations in binary
arithmetic. I imagine it would work for polynomials based on exactly
representable numbers, e.g (x-1.125)^3