-
template<typename T>
void steqr(char *compz, integer *n, T *d, T *e, T *z, integer *ldz, T *work, integer *info)# Tridiagonal QR algorithm.
Purpose:
Computation of all eigenvalues and, optionally, eigenvectors of a symmetric tridiagonal matrix using the implicit QL or QR method. The eigenvectors of a full or band symmetric matrix can also be found if SSYTRD or SSPTRD or SSBTRD has been used to reduce this matrix to tridiagonal form.
- Parameters:
jobz – [in]
jobz is char*
= ‘N’: Compute eigenvalues only.
= ‘V’: Compute eigenvalues and eigenvectors of the original symmetric matrix. On entry, Z must contain the orthogonal matrix used to reduce the original matrix to tridiagonal form.
= ‘I’: Compute eigenvalues and eigenvectors of the tridiagonal matrix. Z is initialized to the identity matrix.
n – [in]
n is integer*
The order of the matrix. n >= 0.
d – [inout]
d is float/double array, dimension (n)
On entry, the diagonal elements of the tridiagonal matrix.
On exit, if info = 0, the eigenvalues in ascending order.e – [inout]
e is float/double array, dimension (n-1)
On entry, the (n-1) subdiagonal elements of the tridiagonal matrix.
On exit, E has been destroyed.z – [inout]
z is float/double array, dimension (ldz, n)
On entry, if jobz = ‘V’, then Z contains the orthogonal matrix used in the reduction to tridiagonal form.
On exit, if info = 0, then if jobz = ‘V’, Z contains the orthonormal eigenvectors of the original symmetric matrix, and if jobz = ‘I’, Z contains the orthonormal eigenvectors of the symmetric tridiagonal matrix.
If jobz = ‘N’, then Z is not referenced.
ldz – [in]
ldz is integer*
The leading dimension of the array Z. ldz >= 1, and if eigenvectors are desired, then ldz >= fla_max(1,n).
WORK – [out]
WORK is REAL array, dimension (fla_max(1,2*N-2))
If COMPZ = ‘N’, then WORK is not referenced.
INFO – [out]
INFO is INTEGER
= 0: successful exit
< 0: if INFO = -i, the i-th argument had an illegal value
> 0: the algorithm has failed to find all the eigenvalues in a total of 30*N iterations; if INFO = i, then i elements of E have not converged to zero; on exit, D and E contain the elements of a symmetric tridiagonal matrix which is unitarily similar to the original matrix.