

 
ZSTEQR2  i a modified version of LAPACK routine ZSTEQR
 SUBROUTINE ZSTEQR2(
 COMPZ, N, D, E, Z, LDZ, NR, WORK, INFO )
CHARACTER COMPZ INTEGER INFO, LDZ, N, NR DOUBLE PRECISION D( * ), E( * ), WORK(
* ) COMPLEX*16 Z( LDZ, * )
ZSTEQR2 is a modified version of LAPACK routine ZSTEQR. ZSTEQR2 computes all
eigenvalues and, optionally, eigenvectors of a symmetric tridiagonal matrix
using the implicit QL or QR method. ZSTEQR2 is modified from ZSTEQR to allow
each ScaLAPACK process running ZSTEQR2 to perform updates on a distributed
matrix Q. Proper usage of ZSTEQR2 can be gleaned from
examination of ScaLAPACK's * PZHEEV.
ZSTEQR2 incorporates changes attributed to Greg Henry.
 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 PZLASET or ZLASET prior to
entering this subroutine.
 N (input) INTEGER
 The order of the matrix. N >= 0.
 D (input/output) DOUBLE PRECISION 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) DOUBLE PRECISION array, dimension (N1)
 On entry, the (n1) subdiagonal elements of the tridiagonal matrix. On
exit, E has been destroyed.
 Z (local input/local output) COMPLEX*16 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) DOUBLE PRECISION array, dimension (max(1,2*N2))
 If COMPZ = 'N', then WORK is not referenced.
 INFO (output) INTEGER
 = 0: successful exit
< 0: if INFO = i, the ith 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.
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