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RedHat 9 (Linux i386) - man page for strexc (redhat section l)

STREXC(l)					)					STREXC(l)

NAME
       STREXC  -  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 STREXC( COMPQ, N, T, LDT, Q, LDQ, IFST, ILST, WORK, INFO )

	   CHARACTER	  COMPQ

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

	   REAL 	  Q( LDQ, * ), T( LDT, * ), WORK( * )

PURPOSE
       STREXC 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 SHSEQR), that is, block upper triangular
       with 1-by-1 and 2-by-2 diagonal blocks; each 2-by-2 diagonal block has its  diagonal  ele-
       ments 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) REAL 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) REAL 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 transposi-
	       tions 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) REAL 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-condi-
	       tioned); 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 				STREXC(l)


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