SYNOPSIS
 SUBROUTINE ZSTEGR(
 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
 DOUBLE PRECISION ABSTOL, VL, VU
 INTEGER ISUPPZ( * ), IWORK( * )
 DOUBLE PRECISION D( * ), E( * ), W( * ), WORK( * )
 COMPLEX*16 Z( LDZ, * )
PURPOSE
ZSTEGR 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.
ZSTEGR is a compatability wrapper around the improved ZSTEMR routine. See DSTEMR for further details.
One important change is that the ABSTOL parameter no longer provides any benefit and hence is no longer used.
Note : ZSTEGR and ZSTEMR work only on machines which follow IEEE754 floatingpoint 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 IEEE754 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 halfopen interval (VL,VU] will be found. = 'I': the ILth through IUth eigenvalues will be found.  N (input) INTEGER
 The order of the matrix. N >= 0.
 D (input/output) DOUBLE PRECISION array, dimension (N)
 On entry, the N diagonal elements of the tridiagonal matrix T. On exit, D is overwritten.
 E (input/output) DOUBLE PRECISION array, dimension (N)
 On entry, the (N1) subdiagonal elements of the tridiagonal matrix T in elements 1 to N1 of E. E(N) need not be set on input, but is used internally as workspace. On exit, E is overwritten.
 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. Not referenced if RANGE = 'A' or 'V'.
 ABSTOL (input) DOUBLE PRECISION
 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 = IUIL+1.
 W (output) DOUBLE PRECISION array, dimension (N)
 The first M elements contain the selected eigenvalues in ascending order.
 Z (output) COMPLEX*16 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 ith 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 ith computed eigenvector is nonzero only in elements ISUPPZ( 2*i1 ) 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) DOUBLE PRECISION 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 ith argument had an illegal value
> 0: if INFO = 1X, internal error in DLARRE, if INFO = 2X, internal error in ZLARRV. Here, the digit X = ABS( IINFO ) < 10, where IINFO is the nonzero error code returned by DLARRE or ZLARRV, respectively.
FURTHER DETAILS
Based on contributions byInderjit Dhillon, IBM Almaden, USA
Osni Marques, LBNL/NERSC, USA
Christof Voemel, LBNL/NERSC, USA