LAPACK  3.4.0
LAPACK: Linear Algebra PACKage
Functions/Subroutines
strexc.f File Reference

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Functions/Subroutines

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

Function/Subroutine Documentation

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]
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:
[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.
[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 >= 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.
Author:
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.
Date:
November 2011

Definition at line 146 of file strexc.f.

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