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NAME
     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, * )



     #include <sunperf.h>

     void ztrexc(char compq, int n, doublecomplex  *t,  int  ldt,
               doublecomplex *q, int ldq, int ifst, int ilst, int
               *info) ;

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
               index IFST is moved to row ILST by a  sequence  of
               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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