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## [Bug-gsl] Problem with gsl_eigen_symmv

 From: Alexandra Bac Subject: [Bug-gsl] Problem with gsl_eigen_symmv Date: Fri, 27 Feb 2009 17:10:40 +0100

```Hi,

I experience the following problem. With the following elementary code :

void Quadriques::dir_courb_principal(gsl_vector *courb, gsl_matrix *dir)
{
gsl_matrix * M = gsl_matrix_alloc(2,2) ;
gsl_matrix_set(M, 0, 0, 2. * this->coef) ;
gsl_matrix_set(M, 0, 1, this->coef) ;
gsl_matrix_set(M, 1, 0, this->coef) ;
gsl_matrix_set(M, 1, 1, 2. * this->coef) ;

gsl_eigen_symmv_workspace * w = gsl_eigen_symmv_alloc (2) ;

// Compute eigenvectors and eigenvalues
gsl_eigen_symmv (M, courb, dir, w) ;
//gsl_eigen_gensymmv_sort (courb, dir, GSL_EIGEN_SORT_VAL_DESC) ;

gsl_matrix_free(M) ;
}

```
called with the following coefs array : [5, -7, 2] (that is, I compute the eigenvalues/
```vectors of the following matrix :
10      -7
-7      2

I get the following eigenvectors :
0.834841        0.550491
-0.550491       0.834841

whereas Maple (and my hand computations) give :

```
Of cours, the sign does not matter, but ...... x and y's are exchanged !!!!!!
```
How is it that such problem occurs ?

Thanks in advance.

Alexandra Bac

```

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