
STGEXC(l) ) STGEXC(l)
NAME
STGEXC  reorder the generalized real Schur decomposition of a real matrix pair (A,B)
using an orthogonal equivalence transformation (A, B) = Q * (A, B) * Z',
SYNOPSIS
SUBROUTINE STGEXC( WANTQ, WANTZ, N, A, LDA, B, LDB, Q, LDQ, Z, LDZ, IFST, ILST, WORK,
LWORK, INFO )
LOGICAL WANTQ, WANTZ
INTEGER IFST, ILST, INFO, LDA, LDB, LDQ, LDZ, LWORK, N
REAL A( LDA, * ), B( LDB, * ), Q( LDQ, * ), WORK( * ), Z( LDZ, * )
PURPOSE
STGEXC reorders the generalized real Schur decomposition of a real matrix pair (A,B) using
an orthogonal equivalence transformation (A, B) = Q * (A, B) * Z', so that the diagonal
block of (A, B) with row index IFST is moved to row ILST.
(A, B) must be in generalized real Schur canonical form (as returned by SGGES), i.e. A is
block upper triangular with 1by1 and 2by2 diagonal blocks. B is upper triangular.
Optionally, the matrices Q and Z of generalized Schur vectors are updated.
Q(in) * A(in) * Z(in)' = Q(out) * A(out) * Z(out)'
Q(in) * B(in) * Z(in)' = Q(out) * B(out) * Z(out)'
ARGUMENTS
WANTQ (input) LOGICAL
WANTZ (input) LOGICAL
N (input) INTEGER
The order of the matrices A and B. N >= 0.
A (input/output) REAL array, dimension (LDA,N)
On entry, the matrix A in generalized real Schur canonical form. On exit, the
updated matrix A, again in generalized real Schur canonical form.
LDA (input) INTEGER
The leading dimension of the array A. LDA >= max(1,N).
B (input/output) REAL array, dimension (LDB,N)
On entry, the matrix B in generalized real Schur canonical form (A,B). On exit,
the updated matrix B, again in generalized real Schur canonical form (A,B).
LDB (input) INTEGER
The leading dimension of the array B. LDB >= max(1,N).
Q (input/output) REAL array, dimension (LDZ,N)
On entry, if WANTQ = .TRUE., the orthogonal matrix Q. On exit, the updated matrix
Q. If WANTQ = .FALSE., Q is not referenced.
LDQ (input) INTEGER
The leading dimension of the array Q. LDQ >= 1. If WANTQ = .TRUE., LDQ >= N.
Z (input/output) REAL array, dimension (LDZ,N)
On entry, if WANTZ = .TRUE., the orthogonal matrix Z. On exit, the updated matrix
Z. If WANTZ = .FALSE., Z is not referenced.
LDZ (input) INTEGER
The leading dimension of the array Z. LDZ >= 1. If WANTZ = .TRUE., LDZ >= N.
IFST (input/output) INTEGER
ILST (input/output) INTEGER Specify the reordering of the diagonal blocks of
(A, B). The block with row index IFST is moved to row ILST, by a sequence of
swapping between adjacent blocks. On exit, if IFST pointed on entry to the second
row of a 2by2 block, it is changed to point to the first row; ILST always points
to the first row of the block in its final position (which may differ from its
input value by +1 or 1). 1 <= IFST, ILST <= N.
WORK (workspace/output) REAL array, dimension (LWORK)
On exit, if INFO = 0, WORK(1) returns the optimal LWORK.
LWORK (input) INTEGER
The dimension of the array WORK. LWORK >= 4*N + 16.
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.
INFO (output) INTEGER
=0: successful exit.
<0: if INFO = i, the ith argument had an illegal value.
=1: The transformed matrix pair (A, B) would be too far from generalized Schur
form; the problem is ill conditioned. (A, B) may have been partially reordered,
and ILST points to the first row of the current position of the block being moved.
FURTHER DETAILS
Based on contributions by
Bo Kagstrom and Peter Poromaa, Department of Computing Science,
Umea University, S901 87 Umea, Sweden.
[1] B. Kagstrom; A Direct Method for Reordering Eigenvalues in the
Generalized Real Schur Form of a Regular Matrix Pair (A, B), in
M.S. Moonen et al (eds), Linear Algebra for Large Scale and
RealTime Applications, Kluwer Academic Publ. 1993, pp 195218.
LAPACK version 3.0 15 June 2000 STGEXC(l) 
