stgexc (l)  Linux Manuals
stgexc: reorders the generalized real Schur decomposition of a real matrix pair (A,B) using an orthogonal equivalence transformation (A, B) = Q * (A, B) * Zaq,
NAME
STGEXC  reorders the generalized real Schur decomposition of a real matrix pair (A,B) using an orthogonal equivalence transformation (A, B) = Q * (A, B) * Zaq,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 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.
ARGUMENTS
 WANTQ (input) LOGICAL .TRUE. : update the left transformation matrix Q;

 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 (MAX(1,LWORK))
 On exit, if INFO = 0, WORK(1) returns the optimal LWORK.
 LWORK (input) INTEGER
 The dimension of the array WORK. LWORK >= 1 when N <= 1, otherwise 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 byBo 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