int
gsl_linalg_symmtd_decomp (gsl_matrix * A, gsl_vector * tau)
{
if (A->size1 != A->size2)
{
GSL_ERROR ("symmetric tridiagonal decomposition requires square matrix",
GSL_ENOTSQR);
}
else if (tau->size + 1 != A->size1)
{
GSL_ERROR ("size of tau must be (matrix size - 1)", GSL_EBADLEN);
}
else
{
const size_t N = A->size1;
size_t i;
for (i = 0 ; i < N - 2; i++)
{
gsl_vector_view c = gsl_matrix_column (A, i);
gsl_vector_view v = gsl_vector_subvector (&c.vector, i + 1, N - (i + 1));
double tau_i = gsl_linalg_householder_transform (&v.vector);
/* Apply the transformation H^T A H to the remaining columns */
if (tau_i != 0.0)
{
gsl_matrix_view m = gsl_matrix_submatrix (A, i + 1, i + 1,
N - (i+1), N - (i+1));
double ei = gsl_vector_get(&v.vector, 0);
gsl_vector_view x = gsl_vector_subvector (tau, i, N-(i+1));
gsl_vector_set (&v.vector, 0, 1.0);
/* x = tau * A * v */
gsl_blas_dsymv (CblasLower, tau_i, &m.matrix, &v.vector, 0.0, &x.vector);
/* w = x - (1/2) tau * (x' * v) * v */
{
double xv, alpha;
gsl_blas_ddot(&x.vector, &v.vector, &xv);
alpha = - (tau_i / 2.0) * xv;
gsl_blas_daxpy(alpha, &v.vector, &x.vector);
}
/* apply the transformation A = A - v w' - w v' */
gsl_blas_dsyr2(CblasLower, -1.0, &v.vector, &x.vector, &m.matrix);
gsl_vector_set (&v.vector, 0, ei);
}
gsl_vector_set (tau, i, tau_i);
}
return GSL_SUCCESS;
}
}
int
gsl_eigen_gensymm_standardize(gsl_matrix *A, const gsl_matrix *B)
{
const size_t N = A->size1;
size_t i;
double a, b, c;
for (i = 0; i < N; ++i)
{
/* update lower triangle of A(i:n, i:n) */
a = gsl_matrix_get(A, i, i);
b = gsl_matrix_get(B, i, i);
a /= b * b;
gsl_matrix_set(A, i, i, a);
if (i < N - 1)
{
gsl_vector_view ai = gsl_matrix_subcolumn(A, i, i + 1, N - i - 1);
gsl_matrix_view ma =
gsl_matrix_submatrix(A, i + 1, i + 1, N - i - 1, N - i - 1);
gsl_vector_const_view bi =
gsl_matrix_const_subcolumn(B, i, i + 1, N - i - 1);
gsl_matrix_const_view mb =
gsl_matrix_const_submatrix(B, i + 1, i + 1, N - i - 1, N - i - 1);
gsl_blas_dscal(1.0 / b, &ai.vector);
c = -0.5 * a;
gsl_blas_daxpy(c, &bi.vector, &ai.vector);
gsl_blas_dsyr2(CblasLower, -1.0, &ai.vector, &bi.vector, &ma.matrix);
gsl_blas_daxpy(c, &bi.vector, &ai.vector);
gsl_blas_dtrsv(CblasLower,
CblasNoTrans,
CblasNonUnit,
&mb.matrix,
&ai.vector);
}
}
return GSL_SUCCESS;
} /* gsl_eigen_gensymm_standardize() */
/**
* C++ version of gsl_blas_dsyr2().
* @param Uplo Upper or lower triangular
* @param alpha A constant
* @param X A vector
* @param Y A vector
* @param A A matrix
* @return Error code on failure
*/
int dsyr2( CBLAS_UPLO_t Uplo, double alpha, vector const& X, vector const& Y, matrix& A ){
return gsl_blas_dsyr2( Uplo, alpha, X.get(), Y.get(), A.get() ); }
