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.

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.


          If N == 0 arguments ILST and IFST may be any value.

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 >= 1, and if


          COMPQ = 'V', 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.

Univ. of Tennessee

Univ. of California Berkeley

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