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

    Univ. of Colorado Denver

    NAG Ltd.

Date:
    November 2011

Definition at line 146 of file strexc.f.

Author

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