LAPACK  3.6.1 LAPACK: Linear Algebra PACKage
 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

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
 [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 quasi-triangular matrix T, in Schur Schur canonical form. On exit, the reordered upper quasi-triangular 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 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.``` [out] WORK ` WORK is DOUBLE PRECISION array, dimension (N)` [out] 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.```
Date
November 2011

Definition at line 148 of file dtrexc.f.

148 *
149 * -- LAPACK computational routine (version 3.4.0) --
150 * -- LAPACK is a software package provided by Univ. of Tennessee, --
151 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
152 * November 2011
153 *
154 * .. Scalar Arguments ..
155  CHARACTER compq
156  INTEGER ifst, ilst, info, ldq, ldt, n
157 * ..
158 * .. Array Arguments ..
159  DOUBLE PRECISION q( ldq, * ), t( ldt, * ), work( * )
160 * ..
161 *
162 * =====================================================================
163 *
164 * .. Parameters ..
165  DOUBLE PRECISION zero
166  parameter ( zero = 0.0d+0 )
167 * ..
168 * .. Local Scalars ..
169  LOGICAL wantq
170  INTEGER here, nbf, nbl, nbnext
171 * ..
172 * .. External Functions ..
173  LOGICAL lsame
174  EXTERNAL lsame
175 * ..
176 * .. External Subroutines ..
177  EXTERNAL dlaexc, xerbla
178 * ..
179 * .. Intrinsic Functions ..
180  INTRINSIC max
181 * ..
182 * .. Executable Statements ..
183 *
184 * Decode and test the input arguments.
185 *
186  info = 0
187  wantq = lsame( compq, 'V' )
188  IF( .NOT.wantq .AND. .NOT.lsame( compq, 'N' ) ) THEN
189  info = -1
190  ELSE IF( n.LT.0 ) THEN
191  info = -2
192  ELSE IF( ldt.LT.max( 1, n ) ) THEN
193  info = -4
194  ELSE IF( ldq.LT.1 .OR. ( wantq .AND. ldq.LT.max( 1, n ) ) ) THEN
195  info = -6
196  ELSE IF( ifst.LT.1 .OR. ifst.GT.n ) THEN
197  info = -7
198  ELSE IF( ilst.LT.1 .OR. ilst.GT.n ) THEN
199  info = -8
200  END IF
201  IF( info.NE.0 ) THEN
202  CALL xerbla( 'DTREXC', -info )
203  RETURN
204  END IF
205 *
206 * Quick return if possible
207 *
208  IF( n.LE.1 )
209  \$ RETURN
210 *
211 * Determine the first row of specified block
212 * and find out it is 1 by 1 or 2 by 2.
213 *
214  IF( ifst.GT.1 ) THEN
215  IF( t( ifst, ifst-1 ).NE.zero )
216  \$ ifst = ifst - 1
217  END IF
218  nbf = 1
219  IF( ifst.LT.n ) THEN
220  IF( t( ifst+1, ifst ).NE.zero )
221  \$ nbf = 2
222  END IF
223 *
224 * Determine the first row of the final block
225 * and find out it is 1 by 1 or 2 by 2.
226 *
227  IF( ilst.GT.1 ) THEN
228  IF( t( ilst, ilst-1 ).NE.zero )
229  \$ ilst = ilst - 1
230  END IF
231  nbl = 1
232  IF( ilst.LT.n ) THEN
233  IF( t( ilst+1, ilst ).NE.zero )
234  \$ nbl = 2
235  END IF
236 *
237  IF( ifst.EQ.ilst )
238  \$ RETURN
239 *
240  IF( ifst.LT.ilst ) THEN
241 *
242 * Update ILST
243 *
244  IF( nbf.EQ.2 .AND. nbl.EQ.1 )
245  \$ ilst = ilst - 1
246  IF( nbf.EQ.1 .AND. nbl.EQ.2 )
247  \$ ilst = ilst + 1
248 *
249  here = ifst
250 *
251  10 CONTINUE
252 *
253 * Swap block with next one below
254 *
255  IF( nbf.EQ.1 .OR. nbf.EQ.2 ) THEN
256 *
257 * Current block either 1 by 1 or 2 by 2
258 *
259  nbnext = 1
260  IF( here+nbf+1.LE.n ) THEN
261  IF( t( here+nbf+1, here+nbf ).NE.zero )
262  \$ nbnext = 2
263  END IF
264  CALL dlaexc( wantq, n, t, ldt, q, ldq, here, nbf, nbnext,
265  \$ work, info )
266  IF( info.NE.0 ) THEN
267  ilst = here
268  RETURN
269  END IF
270  here = here + nbnext
271 *
272 * Test if 2 by 2 block breaks into two 1 by 1 blocks
273 *
274  IF( nbf.EQ.2 ) THEN
275  IF( t( here+1, here ).EQ.zero )
276  \$ nbf = 3
277  END IF
278 *
279  ELSE
280 *
281 * Current block consists of two 1 by 1 blocks each of which
282 * must be swapped individually
283 *
284  nbnext = 1
285  IF( here+3.LE.n ) THEN
286  IF( t( here+3, here+2 ).NE.zero )
287  \$ nbnext = 2
288  END IF
289  CALL dlaexc( wantq, n, t, ldt, q, ldq, here+1, 1, nbnext,
290  \$ work, info )
291  IF( info.NE.0 ) THEN
292  ilst = here
293  RETURN
294  END IF
295  IF( nbnext.EQ.1 ) THEN
296 *
297 * Swap two 1 by 1 blocks, no problems possible
298 *
299  CALL dlaexc( wantq, n, t, ldt, q, ldq, here, 1, nbnext,
300  \$ work, info )
301  here = here + 1
302  ELSE
303 *
304 * Recompute NBNEXT in case 2 by 2 split
305 *
306  IF( t( here+2, here+1 ).EQ.zero )
307  \$ nbnext = 1
308  IF( nbnext.EQ.2 ) THEN
309 *
310 * 2 by 2 Block did not split
311 *
312  CALL dlaexc( wantq, n, t, ldt, q, ldq, here, 1,
313  \$ nbnext, work, info )
314  IF( info.NE.0 ) THEN
315  ilst = here
316  RETURN
317  END IF
318  here = here + 2
319  ELSE
320 *
321 * 2 by 2 Block did split
322 *
323  CALL dlaexc( wantq, n, t, ldt, q, ldq, here, 1, 1,
324  \$ work, info )
325  CALL dlaexc( wantq, n, t, ldt, q, ldq, here+1, 1, 1,
326  \$ work, info )
327  here = here + 2
328  END IF
329  END IF
330  END IF
331  IF( here.LT.ilst )
332  \$ GO TO 10
333 *
334  ELSE
335 *
336  here = ifst
337  20 CONTINUE
338 *
339 * Swap block with next one above
340 *
341  IF( nbf.EQ.1 .OR. nbf.EQ.2 ) THEN
342 *
343 * Current block either 1 by 1 or 2 by 2
344 *
345  nbnext = 1
346  IF( here.GE.3 ) THEN
347  IF( t( here-1, here-2 ).NE.zero )
348  \$ nbnext = 2
349  END IF
350  CALL dlaexc( wantq, n, t, ldt, q, ldq, here-nbnext, nbnext,
351  \$ nbf, work, info )
352  IF( info.NE.0 ) THEN
353  ilst = here
354  RETURN
355  END IF
356  here = here - nbnext
357 *
358 * Test if 2 by 2 block breaks into two 1 by 1 blocks
359 *
360  IF( nbf.EQ.2 ) THEN
361  IF( t( here+1, here ).EQ.zero )
362  \$ nbf = 3
363  END IF
364 *
365  ELSE
366 *
367 * Current block consists of two 1 by 1 blocks each of which
368 * must be swapped individually
369 *
370  nbnext = 1
371  IF( here.GE.3 ) THEN
372  IF( t( here-1, here-2 ).NE.zero )
373  \$ nbnext = 2
374  END IF
375  CALL dlaexc( wantq, n, t, ldt, q, ldq, here-nbnext, nbnext,
376  \$ 1, work, info )
377  IF( info.NE.0 ) THEN
378  ilst = here
379  RETURN
380  END IF
381  IF( nbnext.EQ.1 ) THEN
382 *
383 * Swap two 1 by 1 blocks, no problems possible
384 *
385  CALL dlaexc( wantq, n, t, ldt, q, ldq, here, nbnext, 1,
386  \$ work, info )
387  here = here - 1
388  ELSE
389 *
390 * Recompute NBNEXT in case 2 by 2 split
391 *
392  IF( t( here, here-1 ).EQ.zero )
393  \$ nbnext = 1
394  IF( nbnext.EQ.2 ) THEN
395 *
396 * 2 by 2 Block did not split
397 *
398  CALL dlaexc( wantq, n, t, ldt, q, ldq, here-1, 2, 1,
399  \$ work, info )
400  IF( info.NE.0 ) THEN
401  ilst = here
402  RETURN
403  END IF
404  here = here - 2
405  ELSE
406 *
407 * 2 by 2 Block did split
408 *
409  CALL dlaexc( wantq, n, t, ldt, q, ldq, here, 1, 1,
410  \$ work, info )
411  CALL dlaexc( wantq, n, t, ldt, q, ldq, here-1, 1, 1,
412  \$ work, info )
413  here = here - 2
414  END IF
415  END IF
416  END IF
417  IF( here.GT.ilst )
418  \$ GO TO 20
419  END IF
420  ilst = here
421 *
422  RETURN
423 *
424 * End of DTREXC
425 *
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:62
subroutine dlaexc(WANTQ, N, T, LDT, Q, LDQ, J1, N1, N2, WORK, INFO)
DLAEXC swaps adjacent diagonal blocks of a real upper quasi-triangular matrix in Schur canonical form...
Definition: dlaexc.f:140
logical function lsame(CA, CB)
LSAME
Definition: lsame.f:55

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