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

           Univ. of Colorado Denver

           NAG Ltd.

       Date:
           November 2011

       Definition at line 146 of file dtrexc.f.

Author

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