strexc

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 * 1 COMPQ
  INTEGER N, LDT, LDQ, IFST, ILST, INFO
  REAL T(LDT,*), Q(LDQ,*), WORK(*)
 
  SUBROUTINE STREXC_64( COMPQ, N, T, LDT, Q, LDQ, IFST, ILST, WORK, 
 *      INFO)
  CHARACTER * 1 COMPQ
  INTEGER*8 N, LDT, LDQ, IFST, ILST, INFO
  REAL T(LDT,*), Q(LDQ,*), WORK(*)

F95 INTERFACE

  SUBROUTINE TREXC( COMPQ, N, T, [LDT], Q, [LDQ], IFST, ILST, [WORK], 
 *       [INFO])
  CHARACTER(LEN=1) :: COMPQ
  INTEGER :: N, LDT, LDQ, IFST, ILST, INFO
  REAL, DIMENSION(:) :: WORK
  REAL, DIMENSION(:,:) :: T, Q
 
  SUBROUTINE TREXC_64( COMPQ, N, T, [LDT], Q, [LDQ], IFST, ILST, [WORK], 
 *       [INFO])
  CHARACTER(LEN=1) :: COMPQ
  INTEGER(8) :: N, LDT, LDQ, IFST, ILST, INFO
  REAL, DIMENSION(:) :: WORK
  REAL, DIMENSION(:,:) :: T, Q

C INTERFACE

#include <sunperf.h>

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

void strexc_64(char compq, long n, float *t, long ldt, float *q, long ldq, long *ifst, long *ilst, long *info);

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 elements equal and its off-diagonal elements of opposite sign.

ARGUMENTS

* COMPQ (input)
* N (input): The order of the matrix T. N >= 0.
* T (input/output): 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): The leading dimension of the array T. LDT >= max(1,N).
* Q (input): 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): The leading dimension of the array Q. LDQ >= max(1,N).
* IFST (input/output): 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.
* ILST (input/output): See the description of IFST.
* WORK (workspace): dimension(N)
* INFO (output)