help-gsl
[Top][All Lists]

## [Help-gsl] permutation matrix with lu decomposition problem

 From: Elias Posoukidis Subject: [Help-gsl] permutation matrix with lu decomposition problem Date: Sat, 1 Jun 2019 08:39:45 +0300 User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0

```Hi everyone,

```
i have the following problem with the permutation matrix after a lu -decomposition. The following code
```
//----------------------------------------------

double a_data[] = { 25, 5, 1, 64, 8, 1, 144, 12, 1 };

gsl_matrix_view m = gsl_matrix_view_array(a_data, 3, 3);

int s;

gsl_permutation *p = gsl_permutation_alloc(3);

gsl_linalg_LU_decomp(&m.matrix, p, &s);

for (size_t i = 0; i < 3; i++) {
for (size_t j = 0; j < 3; j++) {
printf("%3.2f,", gsl_matrix_get(&m.matrix, i, j));
}
printf("\n");
}

gsl_permutation_fprintf(stdout, p, " %u");
printf("\n");
gsl_permutation_free(p);

//----------------------------------------------

gives for the lu-matrix

144.00,12.00,1.00,
0.17,2.92,0.83,
0.44,0.91,-0.20,

```
which is the correct result, but for the permutation vector the following  {2 0 1}, which means that
```
the permutation matrix is

0,1,0

0,0,1

1,0,0

but the correct permutation matrix should be

0,0,1

1,0,0

0,1,0

What am i doing wrong?

kind regards,

eliasp

```