void initmatrice(double ** mymat, double n)
{
  int i;
  int j;

  mymat = malloc(sizeof( double *) * n);

 for (i=0; i<n; i++)
   {
     mymat[i]=malloc(sizeof(double) * n);
     for(j=0; i<n; i++)
       {
	 mymat[i][j] = 0.0 ;
       }
   }
}