SSTEQR2 - i a modified version of LAPACK routine SSTEQR
- SUBROUTINE
SSTEQR2(
- COMPZ, N, D, E, Z, LDZ, NR, WORK, INFO )
CHARACTER COMPZ INTEGER INFO, LDZ, N, NR REAL D( * ), E( * ),
WORK( * ), Z( LDZ, * )
SSTEQR2 is a modified version of LAPACK routine SSTEQR. SSTEQR2
computes all eigenvalues and, optionally, eigenvectors of a symmetric
tridiagonal matrix using the implicit QL or QR method. running SSTEQR2 to
perform updates on a distributed matrix Q. Proper usage of SSTEQR2 can be
gleaned from examination of ScaLAPACK's PSSYEV.
- COMPZ (input)
CHARACTER*1
- = 'N': Compute eigenvalues only.
= 'I': Compute eigenvalues and eigenvectors of the tridiagonal matrix. Z
must be initialized to the identity matrix by PDLASET or DLASET prior to
entering this subroutine.
- N (input) INTEGER
- The order of the matrix. N >= 0.
- D (input/output) REAL array,
dimension (N)
- On entry, the diagonal elements of the tridiagonal matrix. On exit, if
INFO = 0, the eigenvalues in ascending order.
- E (input/output) REAL array,
dimension (N-1)
- On entry, the (n-1) subdiagonal elements of the tridiagonal matrix. On
exit, E has been destroyed.
- Z (local input/local output) REAL
array, global
- dimension (N, N), local dimension (LDZ, NR). On entry, if COMPZ = 'V',
then Z contains the orthogonal matrix used in the reduction to tridiagonal
form. On exit, if INFO = 0, then if COMPZ = 'V', Z contains the
orthonormal eigenvectors of the original symmetric matrix, and if COMPZ =
'I', Z contains the orthonormal eigenvectors of the symmetric tridiagonal
matrix. If COMPZ = 'N', then Z is not referenced.
- LDZ (input) INTEGER
- The leading dimension of the array Z. LDZ >= 1, and if eigenvectors are
desired, then LDZ >= max(1,N).
- NR (input) INTEGER
- NR = MAX(1, NUMROC( N, NB, MYPROW, 0, NPROCS ) ). If COMPZ = 'N', then NR
is not referenced.
- WORK (workspace) REAL array,
dimension (max(1,2*N-2))
- If COMPZ = 'N', then WORK is not referenced.
- INFO (output) 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 orthogonally similar to the original matrix.