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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

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