# dtrexc.f(3) [centos man page]

```dtrexc.f(3)							      LAPACK							       dtrexc.f(3)

NAME
dtrexc.f -

SYNOPSIS
Functions/Subroutines
subroutine dtrexc (COMPQ, N, T, LDT, Q, LDQ, IFST, ILST, WORK, INFO)
DTREXC

Function/Subroutine Documentation
subroutine dtrexc (characterCOMPQ, integerN, double precision, dimension( ldt, * )T, integerLDT, double precision, dimension( ldq, * )Q,
integerLDQ, integerIFST, integerILST, double precision, dimension( * )WORK, integerINFO)
DTREXC

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.

Parameters:
COMPQ

COMPQ is CHARACTER*1
= 'V':  update the matrix Q of Schur vectors;
= 'N':  do not update Q.

N

N is INTEGER
The order of the matrix T. N >= 0.

T

T is 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

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

Q

Q is 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

LDQ is INTEGER
The leading dimension of the array Q.  LDQ >= max(1,N).

IFST

IFST is INTEGER

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.

WORK

WORK is DOUBLE PRECISION array, dimension (N)

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.

Author:
Univ. of Tennessee

Univ. of California Berkeley

NAG Ltd.

Date:
November 2011

Definition at line 146 of file dtrexc.f.

Author
Generated automatically by Doxygen for LAPACK from the source code.

Version 3.4.2							  Tue Sep 25 2012						       dtrexc.f(3)```

## Check Out this Related Man Page

```strexc.f(3)							      LAPACK							       strexc.f(3)

NAME
strexc.f -

SYNOPSIS
Functions/Subroutines
subroutine strexc (COMPQ, N, T, LDT, Q, LDQ, IFST, ILST, WORK, INFO)
STREXC

Function/Subroutine Documentation
subroutine strexc (characterCOMPQ, integerN, real, dimension( ldt, * )T, integerLDT, real, dimension( ldq, * )Q, integerLDQ, integerIFST,
integerILST, real, dimension( * )WORK, integerINFO)
STREXC

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.

Parameters:
COMPQ

COMPQ is CHARACTER*1
= 'V':  update the matrix Q of Schur vectors;
= 'N':  do not update Q.

N

N is INTEGER
The order of the matrix T. N >= 0.

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.

LDT

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

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.

LDQ

LDQ is INTEGER
The leading dimension of the array Q.  LDQ >= max(1,N).

IFST

IFST is INTEGER

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.

WORK

WORK is REAL array, dimension (N)

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.

Author:
Univ. of Tennessee

Univ. of California Berkeley

NAG Ltd.

Date:
November 2011

Definition at line 146 of file strexc.f.

Author
Generated automatically by Doxygen for LAPACK from the source code.

Version 3.4.2							  Tue Sep 25 2012						       strexc.f(3)```
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