dtrexc(l) [redhat man page]

DTREXC(l)								 )								 DTREXC(l)

NAME

       DTREXC  -  reorder the real Schur factorization of a real matrix A = Q*T*Q**T, so that the diagonal block of T with row index IFST is moved
       to row ILST

SYNOPSIS

       SUBROUTINE DTREXC( COMPQ, N, T, LDT, Q, LDQ, IFST, ILST, WORK, INFO )

	   CHARACTER	  COMPQ

	   INTEGER	  IFST, ILST, INFO, LDQ, LDT, N

	   DOUBLE	  PRECISION Q( LDQ, * ), T( LDT, * ), WORK( * )

PURPOSE

       DTREXC reorders the real Schur factorization of a real matrix A = Q*T*Q**T, so that the diagonal block of T with row index IFST is moved to
       row  ILST.   The  real  Schur form T is reordered by an orthogonal similarity transformation Z**T*T*Z, and optionally the matrix Q of Schur
       vectors is updated by postmultiplying it with Z.

       T must be in Schur canonical form (as returned by DHSEQR), that is, block upper triangular with 1-by-1 and  2-by-2  diagonal  blocks;  each
       2-by-2 diagonal block has its diagonal elements equal and its off-diagonal elements of opposite sign.

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) DOUBLE PRECISION array, dimension (LDT,N)
	       On  entry,  the	upper  quasi-triangular  matrix  T,  in Schur Schur canonical form.  On exit, the reordered upper quasi-triangular
	       matrix, again in Schur canonical form.

       LDT     (input) INTEGER
	       The leading dimension of the array T. LDT >= max(1,N).

       Q       (input/output) DOUBLE PRECISION 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	orthogonal
	       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/output) INTEGER
	       ILST	(input/output)	INTEGER Specify the reordering of the diagonal blocks of T.  The block with row index IFST is moved to row
	       ILST, by a sequence of transpositions between adjacent blocks.  On exit, if IFST pointed on entry to the second	row  of  a  2-by-2
	       block,  it  is changed to point to the first row; ILST always points to the first row of the block in its final position (which may
	       differ from its input value by +1 or -1).  1 <= IFST <= N; 1 <= ILST <= N.

       WORK    (workspace) DOUBLE PRECISION array, dimension (N)

       INFO    (output) INTEGER
	       = 0:  successful exit
	       < 0:  if INFO = -i, the i-th argument had an illegal value
	       = 1:  two adjacent blocks were too close to swap (the problem is very ill-conditioned); T may have been	partially  reordered,  and
	       ILST points to the first row of the current position of the block being moved.

LAPACK version 3.0						   15 June 2000 							 DTREXC(l)