sstemr (l)  Linux Manuals
sstemr: computes selected eigenvalues and, optionally, eigenvectors of a real symmetric tridiagonal matrix T
NAME
SSTEMR  computes selected eigenvalues and, optionally, eigenvectors of a real symmetric tridiagonal matrix TSYNOPSIS
 SUBROUTINE SSTEMR(
 JOBZ, RANGE, N, D, E, VL, VU, IL, IU, M, W, Z, LDZ, NZC, ISUPPZ, TRYRAC, WORK, LWORK, IWORK, LIWORK, INFO )
 IMPLICIT NONE
 CHARACTER JOBZ, RANGE
 LOGICAL TRYRAC
 INTEGER IL, INFO, IU, LDZ, NZC, LIWORK, LWORK, M, N
 REAL VL, VU
 INTEGER ISUPPZ( * ), IWORK( * )
 REAL D( * ), E( * ), W( * ), WORK( * )
 REAL Z( LDZ, * )
PURPOSE
SSTEMR 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.
Depending on the number of desired eigenvalues, these are computed either by bisection or the dqds algorithm. Numerically orthogonal eigenvectors are computed by the use of various suitable L D L^T factorizations near clusters of close eigenvalues (referred to as RRRs, Relatively Robust Representations). An informal sketch of the algorithm follows. For each unreduced block (submatrix) of T,
(a)
(b)
(c)
(d)
 Inderjit S. Dhillon and Beresford N. Parlett: "Multiple representations
 Inderjit Dhillon: "A new O(n^2) algorithm for the symmetric
Further Details
floatingpoint standard in their handling of infinities and NaNs. This permits the use of efficient inner loops avoiding a check for zero divisors.
ARGUMENTS
 JOBZ (input) CHARACTER*1

= aqNaq: Compute eigenvalues only;
= aqVaq: Compute eigenvalues and eigenvectors.  RANGE (input) CHARACTER*1

= aqAaq: all eigenvalues will be found.
= aqVaq: all eigenvalues in the halfopen interval (VL,VU] will be found. = aqIaq: the ILth through IUth 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 (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) REAL
 VU (input) REAL If RANGE=aqVaq, the lower and upper bounds of the interval to be searched for eigenvalues. VL < VU. Not referenced if RANGE = aqAaq or aqIaq.
 IL (input) INTEGER
 IU (input) INTEGER If RANGE=aqIaq, 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 = aqAaq or aqVaq.
 M (output) INTEGER
 The total number of eigenvalues found. 0 <= M <= N. If RANGE = aqAaq, M = N, and if RANGE = aqIaq, M = IUIL+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 = aqVaq, 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 = aqNaq, 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 = aqVaq, 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 = aqVaq, then LDZ >= max(1,N).
 NZC (input) INTEGER
 The number of eigenvectors to be held in the array Z. If RANGE = aqAaq, then NZC >= max(1,N). If RANGE = aqVaq, then NZC >= the number of eigenvalues in (VL,VU]. If RANGE = aqIaq, then NZC >= IUIL+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 by XERBLA.
 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 aqVaq and N > 0.
 TRYRAC (input/output) LOGICAL
 If TRYRAC.EQ..TRUE., indicates that the code should check whether the tridiagonal matrix defines its eigenvalues to high relative accuracy. If so, the code uses relativeaccuracy preserving algorithms that might be (a bit) slower depending on the matrix. If the matrix does not define its eigenvalues to high relative accuracy, the code can uses possibly faster algorithms. If TRYRAC.EQ..FALSE., the code is not required to guarantee relatively accurate eigenvalues and can use the fastest possible techniques. On exit, a .TRUE. TRYRAC will be set to .FALSE. if the matrix does not define its eigenvalues to high relative accuracy.
 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 = aqVaq, and LWORK >= max(1,12*N) if JOBZ = aqNaq. 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 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 byBeresford Parlett, University of California, Berkeley, USA
Jim Demmel, University of California, Berkeley, USA
Inderjit Dhillon, University of Texas, Austin, USA
Osni Marques, LBNL/NERSC, USA
Christof Voemel, University of California, Berkeley, USA