sstevx
sstevx - compute selected eigenvalues and, optionally, eigenvectors of a real symmetric tridiagonal matrix A
SUBROUTINE SSTEVX( JOBZ, RANGE, N, DIAG, OFFD, VL, VU, IL, IU,
* ABTOL, NFOUND, W, Z, LDZ, WORK, IWORK2, IFAIL, INFO)
CHARACTER * 1 JOBZ, RANGE
INTEGER N, IL, IU, NFOUND, LDZ, INFO
INTEGER IWORK2(*), IFAIL(*)
REAL VL, VU, ABTOL
REAL DIAG(*), OFFD(*), W(*), Z(LDZ,*), WORK(*)
SUBROUTINE SSTEVX_64( JOBZ, RANGE, N, DIAG, OFFD, VL, VU, IL, IU,
* ABTOL, NFOUND, W, Z, LDZ, WORK, IWORK2, IFAIL, INFO)
CHARACTER * 1 JOBZ, RANGE
INTEGER*8 N, IL, IU, NFOUND, LDZ, INFO
INTEGER*8 IWORK2(*), IFAIL(*)
REAL VL, VU, ABTOL
REAL DIAG(*), OFFD(*), W(*), Z(LDZ,*), WORK(*)
SUBROUTINE STEVX( JOBZ, RANGE, N, DIAG, OFFD, VL, VU, IL, IU, ABTOL,
* NFOUND, W, Z, [LDZ], [WORK], [IWORK2], IFAIL, [INFO])
CHARACTER(LEN=1) :: JOBZ, RANGE
INTEGER :: N, IL, IU, NFOUND, LDZ, INFO
INTEGER, DIMENSION(:) :: IWORK2, IFAIL
REAL :: VL, VU, ABTOL
REAL, DIMENSION(:) :: DIAG, OFFD, W, WORK
REAL, DIMENSION(:,:) :: Z
SUBROUTINE STEVX_64( JOBZ, RANGE, N, DIAG, OFFD, VL, VU, IL, IU,
* ABTOL, NFOUND, W, Z, [LDZ], [WORK], [IWORK2], IFAIL, [INFO])
CHARACTER(LEN=1) :: JOBZ, RANGE
INTEGER(8) :: N, IL, IU, NFOUND, LDZ, INFO
INTEGER(8), DIMENSION(:) :: IWORK2, IFAIL
REAL :: VL, VU, ABTOL
REAL, DIMENSION(:) :: DIAG, OFFD, W, WORK
REAL, DIMENSION(:,:) :: Z
#include <sunperf.h>
void sstevx(char jobz, char range, int n, float *diag, float *offd, float vl, float vu, int il, int iu, float abtol, int *nfound, float *w, float *z, int ldz, int *ifail, int *info);
void sstevx_64(char jobz, char range, long n, float *diag, float *offd, float vl, float vu, long il, long iu, float abtol, long *nfound, float *w, float *z, long ldz, long *ifail, long *info);
sstevx computes selected eigenvalues and, optionally, eigenvectors
of a real symmetric tridiagonal matrix A. Eigenvalues and
eigenvectors can be selected by specifying either a range of values
or a range of indices for the desired eigenvalues.
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* JOBZ (input)
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* RANGE (input)
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* N (input)
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The order of the matrix. N >= 0.
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* DIAG (input/output)
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On entry, the n diagonal elements of the tridiagonal matrix
A.
On exit, DIAG may be multiplied by a constant factor chosen
to avoid over/underflow in computing the eigenvalues.
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* OFFD (input/output)
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On entry, the (n-1) subdiagonal elements of the tridiagonal
matrix A in elements 1 to N-1 of OFFD; OFFD(N) need not be set.
On exit, OFFD may be multiplied by a constant factor chosen
to avoid over/underflow in computing the eigenvalues.
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* VL (input)
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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'.
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* VU (input)
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See the description of VL.
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* IL (input)
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If RANGE='I', the indices (in ascending order) of the
smallest and largest eigenvalues to be returned.
1 <= IL <= IU <= N, if N > 0; IL = 1 and IU = 0 if N = 0.
Not referenced if RANGE = 'A' or 'V'.
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* IU (input)
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See the description of IL.
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* ABTOL (input)
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The absolute error tolerance for the eigenvalues.
An approximate eigenvalue is accepted as converged
when it is determined to lie in an interval [a,b]
of width less than or equal to
ABTOL + EPS * max( |a|,|b| ) ,
where EPS is the machine precision. If ABTOL is less
than or equal to zero, then EPS*|T| will be used in
its place, where |T| is the 1-norm of the tridiagonal
matrix.
Eigenvalues will be computed most accurately when ABTOL is
set to twice the underflow threshold 2*SLAMCH('S'), not zero.
If this routine returns with INFO>0, indicating that some
eigenvectors did not converge, try setting ABTOL to
2*SLAMCH('S').
See "Computing Small Singular Values of Bidiagonal Matrices
with Guaranteed High Relative Accuracy," by Demmel and
Kahan, LAPACK Working Note #3.
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* NFOUND (output)
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The total number of eigenvalues found. 0 <= NFOUND <= N.
If RANGE = 'A', NFOUND = N, and if RANGE = 'I', NFOUND = IU-IL+1.
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* W (output)
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The first NFOUND elements contain the selected eigenvalues in
ascending order.
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* Z (input)
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If JOBZ = 'V', then if INFO = 0, the first NFOUND columns of Z
contain the orthonormal eigenvectors of the matrix A
corresponding to the selected eigenvalues, with the i-th
column of Z holding the eigenvector associated with W(i).
If an eigenvector fails to converge (INFO > 0), then that
column of Z contains the latest approximation to the
eigenvector, and the index of the eigenvector is returned
in IFAIL. If JOBZ = 'N', then Z is not referenced.
Note: the user must ensure that at least max(1,NFOUND) columns are
supplied in the array Z; if RANGE = 'V', the exact value of NFOUND
is not known in advance and an upper bound must be used.
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* LDZ (input)
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The leading dimension of the array Z. LDZ >= 1, and if
JOBZ = 'V', LDZ >= max(1,N).
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* WORK (workspace)
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dimension(5*N)
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* IWORK2 (workspace)
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* IFAIL (output)
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If JOBZ = 'V', then if INFO = 0, the first NFOUND elements of
IFAIL are zero. If INFO > 0, then IFAIL contains the
indices of the eigenvectors that failed to converge.
If JOBZ = 'N', then IFAIL is not referenced.
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* INFO (output)
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