LAPACK
3.4.2
LAPACK: Linear Algebra PACKage

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Functions/Subroutines  
subroutine  dtrexc (COMPQ, N, T, LDT, Q, LDQ, IFST, ILST, WORK, INFO) 
DTREXC 
subroutine dtrexc  (  character  COMPQ, 
integer  N,  
double precision, dimension( ldt, * )  T,  
integer  LDT,  
double precision, dimension( ldq, * )  Q,  
integer  LDQ,  
integer  IFST,  
integer  ILST,  
double precision, dimension( * )  WORK,  
integer  INFO  
) 
DTREXC
Download DTREXC + dependencies [TGZ] [ZIP] [TXT]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 1by1 and 2by2 diagonal blocks; each 2by2 diagonal block has its diagonal elements equal and its offdiagonal elements of opposite sign.
[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 DOUBLE PRECISION array, dimension (LDT,N) On entry, the upper quasitriangular matrix T, in Schur Schur canonical form. On exit, the reordered upper quasitriangular 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 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. 
[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 2by2 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 DOUBLE PRECISION array, dimension (N) 
[out]  INFO  INFO is INTEGER = 0: successful exit < 0: if INFO = i, the ith argument had an illegal value = 1: two adjacent blocks were too close to swap (the problem is very illconditioned); T may have been partially reordered, and ILST points to the first row of the current position of the block being moved. 
Definition at line 146 of file dtrexc.f.