ctgexc (l)  Linux Manuals
ctgexc: reorders the generalized Schur decomposition of a complex matrix pair (A,B), using an unitary equivalence transformation (A, B) := Q * (A, B) * Zaq, so that the diagonal block of (A, B) with row index IFST is moved to row ILST
NAME
CTGEXC  reorders the generalized Schur decomposition of a complex matrix pair (A,B), using an unitary equivalence transformation (A, B) := Q * (A, B) * Zaq, so that the diagonal block of (A, B) with row index IFST is moved to row ILSTSYNOPSIS
 SUBROUTINE CTGEXC(
 WANTQ, WANTZ, N, A, LDA, B, LDB, Q, LDQ, Z, LDZ, IFST, ILST, INFO )
 LOGICAL WANTQ, WANTZ
 INTEGER IFST, ILST, INFO, LDA, LDB, LDQ, LDZ, N
 COMPLEX A( LDA, * ), B( LDB, * ), Q( LDQ, * ), Z( LDZ, * )
PURPOSE
CTGEXC reorders the generalized Schur decomposition of a complex matrix pair (A,B), using an unitary equivalence transformation (A, B) := Q * (A, B) * Zaq, so that the diagonal block of (A, B) with row index IFST is moved to row ILST. (A, B) must be in generalized Schur canonical form, that is, A and B are both 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) COMPLEX array, dimension (LDA,N)
 On entry, the upper triangular matrix A in the pair (A, B). On exit, the updated matrix A.
 LDA (input) INTEGER
 The leading dimension of the array A. LDA >= max(1,N).
 B (input/output) COMPLEX array, dimension (LDB,N)
 On entry, the upper triangular matrix B in the pair (A, B). On exit, the updated matrix B.
 LDB (input) INTEGER
 The leading dimension of the array B. LDB >= max(1,N).
 Q (input/output) COMPLEX array, dimension (LDZ,N)
 On entry, if WANTQ = .TRUE., the unitary 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) COMPLEX array, dimension (LDZ,N)
 On entry, if WANTZ = .TRUE., the unitary 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) 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.
 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
[3] B. Kagstrom and P. Poromaa, LAPACKStyle Algorithms and Software