SSTEGR2(3) ScaLAPACK routine of NEC Numeric Library Collection SSTEGR2(3)
NAME
SSTEGR2 - computes selected eigenvalues and, optionally, eigenvectors
of a real symmetric tridiagonal matrix T
SYNOPSIS
SUBROUTINE SSTEGR2( JOBZ, RANGE, N, D, E, VL, VU, IL, IU, M, W, Z, LDZ,
NZC, ISUPPZ, WORK, LWORK, IWORK, LIWORK, DOL, DOU,
ZOFFSET, INFO )
CHARACTER JOBZ, RANGE
INTEGER DOL, DOU, IL, INFO, IU, LDZ, NZC, LIWORK, LWORK, M,
N, ZOFFSET
REAL VL, VU
INTEGER ISUPPZ( * ), IWORK( * )
REAL D( * ), E( * ), W( * ), WORK( * )
REAL Z( LDZ, * )
PURPOSE
SSTEGR2 computes selected eigenvalues and, optionally, eigenvectors of
a real symmetric tridiagonal matrix T. It is invoked in the ScaLAPACK
MRRR driver PDSYEVR and the corresponding Hermitian version either when
only eigenvalues are to be computed, or when only a single processor is
used (the sequential-like case).
SSTEGR2 has been adapted from LAPACK's SSTEGR. Please note the follow-
ing crucial changes.
1. The calling sequence has two additional INTEGER parameters,
DOL and DOU, that should satisfy M>=DOU>=DOL>=1.
SSTEGR2 ONLY computes the eigenpairs
corresponding to eigenvalues DOL through DOU in W. (That is,
instead of computing the eigenpairs belonging to W(1)
through W(M), only the eigenvectors belonging to eigenvalues
W(DOL) through W(DOU) are computed. In this case, only the
eigenvalues DOL:DOU are guaranteed to be fully accurate.
2. M is NOT the number of eigenvalues specified by RANGE, but is
M = DOU - DOL + 1. This concerns the case where only eigenvalues
are computed, but on more than one processor. Thus, in this case
M refers to the number of eigenvalues computed on this processor.
3. The arrays W and Z might not contain all the wanted eigenpairs
locally, instead this information is distributed over other
processors.
ARGUMENTS
JOBZ (input) CHARACTER*1
= 'N': Compute eigenvalues only;
= 'V': Compute eigenvalues and eigenvectors.
RANGE (input) CHARACTER*1
= 'A': all eigenvalues will be found.
= 'V': all eigenvalues in the half-open interval (VL,VU]
will be found.
= 'I': the IL-th through IU-th eigenvalues will be found.
N (input) INTEGER
The order of the matrix. N >= 0.
D (input/output) REAL array, dimension (N)
On entry, the N diagonal elements of the tridiagonal matrix T.
On exit, D is overwritten.
E (input/output) REAL array, dimension (N)
On entry, the (N-1) subdiagonal elements of the tridiagonal
matrix T in elements 1 to N-1 of E. E(N) need not be set on
input, but is used internally as workspace.
On exit, E is overwritten.
VL (input) REAL
VU (input) REAL
If RANGE='V', the lower and upper bounds of the interval to be
searched for eigenvalues. VL < VU.
Not referenced if RANGE = 'A' or 'I'.
IL (input) INTEGER
IU (input) INTEGER
If RANGE='I', the indices (in ascending order) of the smallest
and largest eigenvalues to be returned.
1 <= IL <= IU <= N, if N > 0.
Not referenced if RANGE = 'A' or 'V'.
M (output) INTEGER
Globally summed over all processors, M equals the total number
of eigenvalues found. 0 <= M <= N.
If RANGE = 'A', M = N, and if RANGE = 'I', M = IU-IL+1.
The local output equals M = DOU - DOL + 1.
W (output) REAL array, dimension (N)
The first M elements contain the selected eigenvalues in
ascending order. Note that immediately after exiting this rou-
tine, only the eigenvalues from position DOL:DOU are to reli-
able on this processor because the eigenvalue computation is
done in parallel.
Other processors will hold reliable information on other parts
of the W array. This information is communicated in the ScaLA-
PACK driver.
Z (output) REAL array, dimension (LDZ, max(1,M) )
If JOBZ = 'V', and if INFO = 0, then the first M columns of Z
contain some of the orthonormal eigenvectors of the matrix T
corresponding to the selected eigenvalues, with the i-th column
of Z holding the eigenvector associated with W(i).
If JOBZ = 'N', then Z is not referenced.
Note: the user must ensure that at least max(1,M) columns are
supplied in the array Z; if RANGE = 'V', the exact value of M
is not known in advance and can be computed with a workspace
query by setting NZC = -1, see below.
LDZ (input) INTEGER
The leading dimension of the array Z. LDZ >= 1, and if
JOBZ = 'V', then LDZ >= max(1,N).
NZC (input) INTEGER
The number of eigenvectors to be held in the array Z.
If RANGE = 'A', then NZC >= max(1,N).
If RANGE = 'V', then NZC >= the number of eigenvalues in
(VL,VU].
If RANGE = 'I', then NZC >= IU-IL+1.
If NZC = -1, then a workspace query is assumed; the routine
calculates the number of columns of the array Z that are needed
to hold the eigenvectors.
This value is returned as the first entry of the Z array, and
no error message related to NZC is issued.
ISUPPZ (output) INTEGER ARRAY, dimension ( 2*max(1,M) )
The support of the eigenvectors in Z, i.e., the indices indi-
cating the nonzero elements in Z. The i-th computed eigenvector
is nonzero only in elements ISUPPZ( 2*i-1 ) through ISUPPZ( 2*i
). This is relevant in the case when the matrix is split.
ISUPPZ is only set if N>2.
WORK (workspace/output) REAL array, dimension (LWORK)
On exit, if INFO = 0, WORK(1) returns the optimal (and minimal)
LWORK.
LWORK (input) INTEGER
The dimension of the array WORK. LWORK >= max(1,18*N) if JOBZ =
'V', and LWORK >= max(1,12*N) if JOBZ = 'N'.
If LWORK = -1, then a workspace query is assumed; the routine
only calculates the optimal size of the WORK array, returns
this value as the first entry of the WORK array, and no error
message related to LWORK is issued.
IWORK (workspace/output) INTEGER array, dimension (LIWORK)
On exit, if INFO = 0, IWORK(1) returns the optimal LIWORK.
LIWORK (input) INTEGER
The dimension of the array IWORK. LIWORK >= max(1,10*N) if the
eigenvectors are desired, and LIWORK >= max(1,8*N) if only the
eigenvalues are to be computed.
If LIWORK = -1, then a workspace query is assumed; the routine
only calculates the optimal size of the IWORK array, returns
this value as the first entry of the IWORK array, and no error
message related to LIWORK is issued.
DOL (input) INTEGER
DOU (input) INTEGER
From the eigenvalues W(1:M), only eigenvectors Z(:,DOL) to
Z(:,DOU) are computed.
If DOL > 1, then Z(:,DOL-1-ZOFFSET) is used and overwritten.
If DOU < M, then Z(:,DOU+1-ZOFFSET) is used and overwritten.
ZOFFSET (input) INTEGER
Offset for storing the eigenpairs when Z is distributed in 1D-
cyclic fashion
INFO (output) INTEGER
On exit, INFO
= 0: successful exit
other:if INFO = -i, the i-th argument had an illegal value
if INFO = 10X, internal error in SLARRE2,
if INFO = 20X, internal error in SLARRV.
Here, the digit X = ABS( IINFO ) < 10, where IINFO is
the nonzero error code returned by SLARRE2 or
SLARRV, respectively.
ScaLAPACK routine 31 October 2017 SSTEGR2(3)