DSYEVX(3)
computes selected eigenvalues and, optionally, eigenvectors of a real symmetric matrix A
SYNOPSIS
 SUBROUTINE DSYEVX(

JOBZ, RANGE, UPLO, N, A, LDA, VL, VU, IL, IU,
ABSTOL, M, W, Z, LDZ, WORK, LWORK, IWORK,
IFAIL, INFO )

CHARACTER
JOBZ, RANGE, UPLO

INTEGER
IL, INFO, IU, LDA, LDZ, LWORK, M, N

DOUBLE
PRECISION ABSTOL, VL, VU

INTEGER
IFAIL( * ), IWORK( * )

DOUBLE
PRECISION A( LDA, * ), W( * ), WORK( * ), Z( LDZ, * )
PURPOSE
DSYEVX computes selected eigenvalues and, optionally, eigenvectors
of a real symmetric matrix A. Eigenvalues and eigenvectors can be
selected by specifying either a range of values or a range of indices
for the desired eigenvalues.
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 halfopen interval (VL,VU]
will be found.
= 'I': the ILth through IUth eigenvalues will be found.
 UPLO (input) CHARACTER*1

= 'U': Upper triangle of A is stored;
= 'L': Lower triangle of A is stored.
 N (input) INTEGER

The order of the matrix A. N >= 0.
 A (input/output) DOUBLE PRECISION array, dimension (LDA, N)

On entry, the symmetric matrix A. If UPLO = 'U', the
leading NbyN upper triangular part of A contains the
upper triangular part of the matrix A. If UPLO = 'L',
the leading NbyN lower triangular part of A contains
the lower triangular part of the matrix A.
On exit, the lower triangle (if UPLO='L') or the upper
triangle (if UPLO='U') of A, including the diagonal, is
destroyed.
 LDA (input) INTEGER

The leading dimension of the array A. LDA >= max(1,N).
 VL (input) DOUBLE PRECISION

VU (input) DOUBLE PRECISION
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; IL = 1 and IU = 0 if N = 0.
Not referenced if RANGE = 'A' or 'V'.
 ABSTOL (input) DOUBLE PRECISION

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
ABSTOL + EPS * max( a,b ) ,
where EPS is the machine precision. If ABSTOL is less than
or equal to zero, then EPS*T will be used in its place,
where T is the 1norm of the tridiagonal matrix obtained
by reducing A to tridiagonal form.
Eigenvalues will be computed most accurately when ABSTOL is
set to twice the underflow threshold 2*DLAMCH('S'), not zero.
If this routine returns with INFO>0, indicating that some
eigenvectors did not converge, try setting ABSTOL to
2*DLAMCH('S').
See "Computing Small Singular Values of Bidiagonal Matrices
with Guaranteed High Relative Accuracy," by Demmel and
Kahan, LAPACK Working Note #3.
 M (output) INTEGER

The total number of eigenvalues found. 0 <= M <= N.
If RANGE = 'A', M = N, and if RANGE = 'I', M = IUIL+1.
 W (output) DOUBLE PRECISION array, dimension (N)

On normal exit, the first M elements contain the selected
eigenvalues in ascending order.
 Z (output) DOUBLE PRECISION array, dimension (LDZ, max(1,M))

If JOBZ = 'V', then if INFO = 0, the first M columns of Z
contain the orthonormal eigenvectors of the matrix A
corresponding to the selected eigenvalues, with the ith
column of Z holding the eigenvector associated with W(i).
If an eigenvector fails to converge, 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,M) columns are
supplied in the array Z; if RANGE = 'V', the exact value of M
is not known in advance and an upper bound must be used.
 LDZ (input) INTEGER

The leading dimension of the array Z. LDZ >= 1, and if
JOBZ = 'V', LDZ >= max(1,N).
 WORK (workspace/output) DOUBLE PRECISION array, dimension (MAX(1,LWORK))

On exit, if INFO = 0, WORK(1) returns the optimal LWORK.
 LWORK (input) INTEGER

The length of the array WORK. LWORK >= 1, when N <= 1;
otherwise 8*N.
For optimal efficiency, LWORK >= (NB+3)*N,
where NB is the max of the blocksize for DSYTRD and DORMTR
returned by ILAENV.
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 by XERBLA.
 IWORK (workspace) INTEGER array, dimension (5*N)

 IFAIL (output) INTEGER array, dimension (N)

If JOBZ = 'V', then if INFO = 0, the first M 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.
 INFO (output) INTEGER

= 0: successful exit
< 0: if INFO = i, the ith argument had an illegal value
> 0: if INFO = i, then i eigenvectors failed to converge.
Their indices are stored in array IFAIL.