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LAPACK 3.11.0
LAPACK: Linear Algebra PACKage
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LAPACK 3.11.0
LAPACK: Linear Algebra PACKage
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| subroutine strexc | ( | character | COMPQ, |
| integer | N, | ||
| real, dimension( ldt, * ) | T, | ||
| integer | LDT, | ||
| real, dimension( ldq, * ) | Q, | ||
| integer | LDQ, | ||
| integer | IFST, | ||
| integer | ILST, | ||
| real, dimension( * ) | WORK, | ||
| integer | INFO | ||
| ) |
STREXC
Download STREXC + dependencies [TGZ] [ZIP] [TXT]
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.
| [in] | COMPQ | COMPQ is CHARACTER*1
= 'V': update the matrix Q of Schur vectors;
= 'N': do not update Q. |
| [in] | N | N is INTEGER
The order of the matrix T. N >= 0.
If N == 0 arguments ILST and IFST may be any value. |
| [in,out] | T | T is 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. |
| [in] | LDT | LDT is INTEGER
The leading dimension of the array T. LDT >= max(1,N). |
| [in,out] | Q | Q is 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. |
| [in] | LDQ | LDQ is INTEGER
The leading dimension of the array Q. LDQ >= 1, and if
COMPQ = 'V', LDQ >= max(1,N). |
| [in,out] | IFST | IFST is INTEGER |
| [in,out] | ILST | ILST is 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. |
| [out] | WORK | WORK is REAL array, dimension (N) |
| [out] | INFO | INFO is 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. |
1.9.4