Fwd: [Help-gsl] gsl_poly_solve_cubic problem

[nick kama]

Hello,
Thank you for your answer, i think this bad behaviour  would need some more
attention.
This is an unphysical situation, it rarely  happens to have a diagonal
matrix with two  or more equal double, but it could happen and i think this
is an unwanted behaviour and should be fixed or documented in some-way.

Niccolo'


On Dec 20, 2007 3:15 AM, Ralph Silva <address@hidden> wrote:

> Hi Niccolo,
>
> it looks like a kind of round error of the *gsl_poly_solve_cubic* . I have
> tested your program with other GSL functions and they gave quite the same
> results. Take a look at the *gsl_poly_complex_solve_cubic *and *
> gsl_poly_complex_solve*. You will see nearly the expected result.
>
> Cheers,
> Ralph.
>
>  gcc -static test.c -lgsl -lm
>  GSL 1.10
>  Fedora 8 - Kernel 2.6.23.8-63.fc8
>  gcc 4.1.2 20070925 (Red Hat 4.1.2-33)
>  Intel Core 2 Quad
>
>
> On Dec 20, 2007 11:34 AM, nick kama <address@hidden > wrote:
>
> > Hello,
> >
> > I have this problem with the solver of the cubic equations in gsl:
> >
> > if i take  an equation of the form
> >
> > (x-1)^3=0
> >
> > solver finds 3 roots
> > 1 1 1
> >
> > if i took the equation
> > (x-1.1)^3
> >
> > it finds only one root equal to 1.1,this is a little of code  that can
> > show
> > this behaviour.
> > Documentation says :"the case of coincident roots is not considered
> > special.
> > For example, the equation (x-1)^3=0 will have three roots with exactly
> > equal
> > values " so this should'nt happen, pleas let me know where i'm wronging.
> >
> > Many Thanks
> >
> > Niccolo'
> >
> >
> > #include <iostream>
> > #include <gsl/gsl_matrix.h>
> > #include <gsl/gsl_eigen.h>
> > #include <gsl/gsl_vector.h>
> > #include <gsl/gsl_poly.h>
> >
> >
> >
> > main()
> > {
> >
> >  double landa1=0;
> >  double landa2=0;
> >  double landa3=0;
> >  //(x-1.1)^3
> >  int numero = gsl_poly_solve_cubic ( - 3.3,3.63, -1.331, &landa1,
> > &landa2,
> > &landa3);
> >  std::cout << numero<< std::endl;
> >  std::cout << landa1<< std::endl;
> >  std::cout << landa2<< std::endl;
> >  std::cout << landa3<< std::endl;
> >
> >  // (x-1)^3
> >  numero = gsl_poly_solve_cubic ( -3,3, -1, &landa1, &landa2, &landa3);
> >  std::cout << numero<< std::endl;
> >  std::cout << landa1<< std::endl;
> >  std::cout << landa2<< std::endl;
> >  std::cout << landa3<< std::endl;
> >
> > }
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> >
>
>