strexc.f man page

strexc.f —



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

Function/Subroutine Documentation

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



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


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


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


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


          IFST is INTEGER


          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 is REAL array, dimension (N)


          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

Univ. of Colorado Denver

NAG Ltd.


November 2011

Definition at line 146 of file strexc.f.


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Referenced By

strexc(3) is an alias of strexc.f(3).

Sat Nov 16 2013 Version 3.4.2 LAPACK