STRSEN(1) LAPACK routine (version 3.2) STRSEN(1)NAME
STRSEN - reorders the real Schur factorization of a real matrix A =
Q*T*Q**T, so that a selected cluster of eigenvalues appears in the
leading diagonal blocks of the upper quasi-triangular matrix T,
SYNOPSIS
SUBROUTINE STRSEN( JOB, COMPQ, SELECT, N, T, LDT, Q, LDQ, WR, WI, M, S,
SEP, WORK, LWORK, IWORK, LIWORK, INFO )
CHARACTER COMPQ, JOB
INTEGER INFO, LDQ, LDT, LIWORK, LWORK, M, N
REAL S, SEP
LOGICAL SELECT( * )
INTEGER IWORK( * )
REAL Q( LDQ, * ), T( LDT, * ), WI( * ), WORK( * ), WR( *
)
PURPOSE
STRSEN reorders the real Schur factorization of a real matrix A =
Q*T*Q**T, so that a selected cluster of eigenvalues appears in the
leading diagonal blocks of the upper quasi-triangular matrix T, and the
leading columns of Q form an orthonormal basis of the corresponding
right invariant subspace.
Optionally the routine computes the reciprocal condition numbers of the
cluster of eigenvalues and/or the invariant subspace. T must be in
Schur canonical form (as returned by SHSEQR), that is, block upper tri‐
angular with 1-by-1 and 2-by-2 diagonal blocks; each 2-by-2 diagonal
block has its diagonal elemnts equal and its off-diagonal elements of
opposite sign.
ARGUMENTS
JOB (input) CHARACTER*1
Specifies whether condition numbers are required for the clus‐
ter of eigenvalues (S) or the invariant subspace (SEP):
= 'N': none;
= 'E': for eigenvalues only (S);
= 'V': for invariant subspace only (SEP);
= 'B': for both eigenvalues and invariant subspace (S and SEP).
COMPQ (input) CHARACTER*1
= 'V': update the matrix Q of Schur vectors;
= 'N': do not update Q.
SELECT (input) LOGICAL array, dimension (N)
SELECT specifies the eigenvalues in the selected cluster. To
select a real eigenvalue w(j), SELECT(j) must be set to .TRUE..
To select a complex conjugate pair of eigenvalues w(j) and
w(j+1), corresponding to a 2-by-2 diagonal block, either
SELECT(j) or SELECT(j+1) or both must be set to .TRUE.; a com‐
plex conjugate pair of eigenvalues must be either both included
in the cluster or both excluded.
N (input) INTEGER
The order of the matrix T. N >= 0.
T (input/output) REAL array, dimension (LDT,N)
On entry, the upper quasi-triangular matrix T, in Schur canoni‐
cal form. On exit, T is overwritten by the reordered matrix T,
again in Schur canonical form, with the selected eigenvalues in
the leading diagonal blocks.
LDT (input) INTEGER
The leading dimension of the array T. LDT >= max(1,N).
Q (input/output) REAL array, dimension (LDQ,N)
On entry, if COMPQ = 'V', the matrix Q of Schur vectors. On
exit, if COMPQ = 'V', Q has been postmultiplied by the orthogo‐
nal transformation matrix which reorders T; the leading M col‐
umns of Q form an orthonormal basis for the specified invariant
subspace. If COMPQ = 'N', Q is not referenced.
LDQ (input) INTEGER
The leading dimension of the array Q. LDQ >= 1; and if COMPQ =
'V', LDQ >= N.
WR (output) REAL array, dimension (N)
WI (output) REAL array, dimension (N) The real and imagi‐
nary parts, respectively, of the reordered eigenvalues of T.
The eigenvalues are stored in the same order as on the diagonal
of T, with WR(i) = T(i,i) and, if T(i:i+1,i:i+1) is a 2-by-2
diagonal block, WI(i) > 0 and WI(i+1) = -WI(i). Note that if a
complex eigenvalue is sufficiently ill-conditioned, then its
value may differ significantly from its value before reorder‐
ing.
M (output) INTEGER
The dimension of the specified invariant subspace. 0 < = M <=
N.
S (output) REAL
If JOB = 'E' or 'B', S is a lower bound on the reciprocal con‐
dition number for the selected cluster of eigenvalues. S can‐
not underestimate the true reciprocal condition number by more
than a factor of sqrt(N). If M = 0 or N, S = 1. If JOB = 'N'
or 'V', S is not referenced.
SEP (output) REAL
If JOB = 'V' or 'B', SEP is the estimated reciprocal condition
number of the specified invariant subspace. If M = 0 or N, SEP
= norm(T). If JOB = 'N' or 'E', SEP is not referenced.
WORK (workspace/output) REAL array, dimension (MAX(1,LWORK))
On exit, if INFO = 0, WORK(1) returns the optimal LWORK.
LWORK (input) INTEGER
The dimension of the array WORK. If JOB = 'N', LWORK >=
max(1,N); if JOB = 'E', LWORK >= max(1,M*(N-M)); if JOB = 'V'
or 'B', LWORK >= max(1,2*M*(N-M)). 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 (MAX(1,LIWORK))
On exit, if INFO = 0, IWORK(1) returns the optimal LIWORK.
LIWORK (input) INTEGER
The dimension of the array IWORK. If JOB = 'N' or 'E', LIWORK
>= 1; if JOB = 'V' or 'B', LIWORK >= max(1,M*(N-M)). If LIWORK
= -1, then a workspace query is assumed; the routine only cal‐
culates 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
= 0: successful exit
< 0: if INFO = -i, the i-th argument had an illegal value
= 1: reordering of T failed because some eigenvalues are too
close to separate (the problem is very ill-conditioned); T may
have been partially reordered, and WR and WI contain the eigen‐
values in the same order as in T; S and SEP (if requested) are
set to zero.
FURTHER DETAILS
STRSEN first collects the selected eigenvalues by computing an orthogo‐
nal transformation Z to move them to the top left corner of T. In
other words, the selected eigenvalues are the eigenvalues of T11 in:
Z'*T*Z = ( T11 T12 ) n1
( 0 T22 ) n2
n1 n2
where N = n1+n2 and Z' means the transpose of Z. The first n1 columns
of Z span the specified invariant subspace of T.
If T has been obtained from the real Schur factorization of a matrix A
= Q*T*Q', then the reordered real Schur factorization of A is given by
A = (Q*Z)*(Z'*T*Z)*(Q*Z)', and the first n1 columns of Q*Z span the
corresponding invariant subspace of A.
The reciprocal condition number of the average of the eigenvalues of
T11 may be returned in S. S lies between 0 (very badly conditioned) and
1 (very well conditioned). It is computed as follows. First we compute
R so that
P = ( I R ) n1
( 0 0 ) n2
n1 n2
is the projector on the invariant subspace associated with T11. R is
the solution of the Sylvester equation:
T11*R - R*T22 = T12.
Let F-norm(M) denote the Frobenius-norm of M and 2-norm(M) denote the
two-norm of M. Then S is computed as the lower bound
(1 + F-norm(R)**2)**(-1/2)
on the reciprocal of 2-norm(P), the true reciprocal condition number.
S cannot underestimate 1 / 2-norm(P) by more than a factor of sqrt(N).
An approximate error bound for the computed average of the eigenvalues
of T11 is
EPS * norm(T) / S
where EPS is the machine precision.
The reciprocal condition number of the right invariant subspace spanned
by the first n1 columns of Z (or of Q*Z) is returned in SEP. SEP is
defined as the separation of T11 and T22:
sep( T11, T22 ) = sigma-min( C )
where sigma-min(C) is the smallest singular value of the
n1*n2-by-n1*n2 matrix
C = kprod( I(n2), T11 ) - kprod( transpose(T22), I(n1) ) I(m) is an
m by m identity matrix, and kprod denotes the Kronecker product. We
estimate sigma-min(C) by the reciprocal of an estimate of the 1-norm of
inverse(C). The true reciprocal 1-norm of inverse(C) cannot differ from
sigma-min(C) by more than a factor of sqrt(n1*n2). When SEP is small,
small changes in T can cause large changes in the invariant subspace.
An approximate bound on the maximum angular error in the computed right
invariant subspace is
EPS * norm(T) / SEP
LAPACK routine (version 3.2) November 2008 STRSEN(1)
