ztrexc(3)

NAME

ZTREXC - reorder the Schur factorization of a complex ma

trix A = Q*T*Q**H, so that the diagonal element of T with row in

dex IFST is moved to row ILST

SYNOPSIS

SUBROUTINE ZTREXC( COMPQ, N, T, LDT, Q, LDQ,  IFST,  ILST,
INFO )
    CHARACTER      COMPQ
    INTEGER        IFST, ILST, INFO, LDQ, LDT, N
    COMPLEX*16     Q( LDQ, * ), T( LDT, * )

PURPOSE

ZTREXC reorders the Schur factorization of a complex ma

trix A = Q*T*Q**H, so that the diagonal element of T with row in

dex 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*16 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*16 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 index IFST is

moved to row ILST by a sequence of transpositions between adja

cent 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

LAPACK version 3.0 15 June 2000

ZTREXC(3)