SSTEGR(3f)

NAME
SSTEGR - selected eigenvalues and, optionally, eigenvectors of a real symmetric tridiagonal matrix T

SYNOPSIS

SUBROUTINE SSTEGR(
    JOBZ, RANGE, N, D, E, VL, VU, IL, IU, ABSTOL, M, W, Z, LDZ, ISUPPZ, WORK, LWORK, IWORK, LIWORK, INFO )

IMPLICIT
    NONE

CHARACTER
    JOBZ, RANGE

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

REAL
    ABSTOL, VL, VU

INTEGER
    ISUPPZ( * ), IWORK( * )

REAL
    D( * ), E( * ), W( * ), WORK( * )

REAL
    Z( LDZ, * )

PURPOSE
SSTEGR computes selected eigenvalues and, optionally, eigenvectors of a real symmetric tridiagonal matrix T. Any such unreduced matrix has a well defined set of pairwise different real eigenvalues, the corresponding real eigenvectors are pairwise orthogonal.
The spectrum may be computed either completely or partially by specifying either an interval (VL,VU] or a range of indices IL:IU for the desired eigenvalues.
SSTEGR is a compatability wrapper around the improved SSTEMR routine. See SSTEMR for further details.
One important change is that the ABSTOL parameter no longer provides any benefit and hence is no longer used.
Note : SSTEGR and SSTEMR work only on machines which follow IEEE-754 floating-point standard in their handling of infinities and NaNs. Normal execution may create these exceptiona values and hence may abort due to a floating point exception in environments which do not conform to the IEEE-754 standard.

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'.

ABSTOL  (input) REAL
    Unused. Was the absolute error tolerance for the eigenvalues/eigenvectors in previous versions.

M       (output) INTEGER
    The total number of eigenvalues found. 0 <= M <= N. If RANGE = 'A', M = N, and if RANGE = 'I', M = IU-IL+1.

W       (output) REAL array, dimension (N)
    The first M elements contain the selected eigenvalues in ascending order.

Z       (output) REAL array, dimension (LDZ, max(1,M) )
    If JOBZ = 'V', and if INFO = 0, then the first M columns of Z contain 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 an upper bound must be used. Supplying N columns is always safe.

LDZ     (input) INTEGER
    The leading dimension of the array Z. LDZ >= 1, and if JOBZ = 'V', then LDZ >= max(1,N).

ISUPPZ  (output) INTEGER ARRAY, dimension ( 2*max(1,M) )
    The support of the eigenvectors in Z, i.e., the indices indicating 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 accessed when JOBZ is 'V' and N > 0.

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 by XERBLA.

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 by XERBLA.

INFO    (output) INTEGER
    On exit, INFO = 0: successful exit
    < 0: if INFO = -i, the i-th argument had an illegal value
    > 0: if INFO = 1X, internal error in SLARRE, if INFO = 2X, internal error in SLARRV. Here, the digit X = ABS( IINFO ) < 10, where IINFO is the nonzero error code returned by SLARRE or SLARRV, respectively.

FURTHER DETAILS
Based on contributions by
   Inderjit Dhillon, IBM Almaden, USA
   Osni Marques, LBNL/NERSC, USA
   Christof Voemel, LBNL/NERSC, USA