CTREXC - reorder the Schur factorization of a complex
matrix A = Q*T*Q**H, so that the diagonal element of T
with row index IFST is moved to row ILST
SYNOPSIS
SUBROUTINE CTREXC( COMPQ, N, T, LDT, Q, LDQ, IFST, ILST,
INFO )
CHARACTER COMPQ
INTEGER IFST, ILST, INFO, LDQ, LDT, N
COMPLEX Q( LDQ, * ), T( LDT, * )
PURPOSE
CTREXC reorders the Schur factorization of a complex
matrix A = Q*T*Q**H, so that the diagonal element of T
with row index IFST is moved to row ILST. The Schur form
T is reordered by a unitary similarity transformation
Z**H*T*Z, and optionally the matrix Q of Schur vectors is
updated by postmultplying it with Z.
ARGUMENTS
COMPQ (input) CHARACTER*1
= 'V': update the matrix Q of Schur vectors;
= 'N': do not update Q.
N (input) INTEGER
The order of the matrix T. N >= 0.
T (input/output) COMPLEX array, dimension (LDT,N)
On entry, the upper triangular matrix T. On exit,
the reordered upper triangular matrix.
LDT (input) INTEGER
The leading dimension of the array T. LDT >=
max(1,N).
Q (input/output) COMPLEX 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 unitary transformation
matrix Z which reorders T. If COMPQ = 'N', Q is
not referenced.
LDQ (input) INTEGER
The leading dimension of the array Q. LDQ >=
max(1,N).
IFST (input) INTEGER
ILST (input) INTEGER Specify the reordering of
the diagonal elements of T: The element with row
transpositions between adjacent elements. 1 <=
IFST <= N; 1 <= ILST <= N.
INFO (output) INTEGER
= 0: successful exit
< 0: if INFO = -i, the i-th argument had an ille
gal value
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