ZTREXC  -  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 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
       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*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

               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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