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
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(*)
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
#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);
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.
= 'V': update the matrix Q of Schur vectors;
= 'N': do not update Q.
dimension(N)
= 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.