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---
:name: strexc
:md5sum: a068be55bb498b1fa71d32054496ba06
:category: :subroutine
:arguments:
- compq:
:type: char
:intent: input
- n:
:type: integer
:intent: input
- t:
:type: real
:intent: input/output
:dims:
- ldt
- n
- ldt:
:type: integer
:intent: input
- q:
:type: real
:intent: input/output
:dims:
- ldq
- n
- ldq:
:type: integer
:intent: input
- ifst:
:type: integer
:intent: input/output
- ilst:
:type: integer
:intent: input/output
- work:
:type: real
:intent: workspace
:dims:
- n
- info:
:type: integer
:intent: output
:substitutions: {}
:fortran_help: " SUBROUTINE STREXC( COMPQ, N, T, LDT, Q, LDQ, IFST, ILST, WORK, INFO )\n\n\
* Purpose\n\
* =======\n\
*\n\
* STREXC reorders the real Schur factorization of a real matrix\n\
* A = Q*T*Q**T, so that the diagonal block of T with row index IFST is\n\
* moved to row ILST.\n\
*\n\
* The real Schur form T is reordered by an orthogonal similarity\n\
* transformation Z**T*T*Z, and optionally the matrix Q of Schur vectors\n\
* is updated by postmultiplying it with Z.\n\
*\n\
* T must be in Schur canonical form (as returned by SHSEQR), that is,\n\
* block upper triangular with 1-by-1 and 2-by-2 diagonal blocks; each\n\
* 2-by-2 diagonal block has its diagonal elements equal and its\n\
* off-diagonal elements of opposite sign.\n\
*\n\n\
* Arguments\n\
* =========\n\
*\n\
* COMPQ (input) CHARACTER*1\n\
* = 'V': update the matrix Q of Schur vectors;\n\
* = 'N': do not update Q.\n\
*\n\
* N (input) INTEGER\n\
* The order of the matrix T. N >= 0.\n\
*\n\
* T (input/output) REAL array, dimension (LDT,N)\n\
* On entry, the upper quasi-triangular matrix T, in Schur\n\
* Schur canonical form.\n\
* On exit, the reordered upper quasi-triangular matrix, again\n\
* in Schur canonical form.\n\
*\n\
* LDT (input) INTEGER\n\
* The leading dimension of the array T. LDT >= max(1,N).\n\
*\n\
* Q (input/output) REAL array, dimension (LDQ,N)\n\
* On entry, if COMPQ = 'V', the matrix Q of Schur vectors.\n\
* On exit, if COMPQ = 'V', Q has been postmultiplied by the\n\
* orthogonal transformation matrix Z which reorders T.\n\
* If COMPQ = 'N', Q is not referenced.\n\
*\n\
* LDQ (input) INTEGER\n\
* The leading dimension of the array Q. LDQ >= max(1,N).\n\
*\n\
* IFST (input/output) INTEGER\n\
* ILST (input/output) INTEGER\n\
* Specify the reordering of the diagonal blocks of T.\n\
* The block with row index IFST is moved to row ILST, by a\n\
* sequence of transpositions between adjacent blocks.\n\
* On exit, if IFST pointed on entry to the second row of a\n\
* 2-by-2 block, it is changed to point to the first row; ILST\n\
* always points to the first row of the block in its final\n\
* position (which may differ from its input value by +1 or -1).\n\
* 1 <= IFST <= N; 1 <= ILST <= N.\n\
*\n\
* WORK (workspace) REAL array, dimension (N)\n\
*\n\
* INFO (output) INTEGER\n\
* = 0: successful exit\n\
* < 0: if INFO = -i, the i-th argument had an illegal value\n\
* = 1: two adjacent blocks were too close to swap (the problem\n\
* is very ill-conditioned); T may have been partially\n\
* reordered, and ILST points to the first row of the\n\
* current position of the block being moved.\n\
*\n\n\
* =====================================================================\n\
*\n"
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