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stgsy2.f
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1 *> \brief \b STGSY2 solves the generalized Sylvester equation (unblocked algorithm).
2 *
3 * =========== DOCUMENTATION ===========
4 *
5 * Online html documentation available at
6 * http://www.netlib.org/lapack/explore-html/
7 *
8 *> \htmlonly
9 *> Download STGSY2 + dependencies
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11 *> [TGZ]</a>
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13 *> [ZIP]</a>
14 *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.txt?format=txt&filename=/lapack/lapack_routine/stgsy2.f">
15 *> [TXT]</a>
16 *> \endhtmlonly
17 *
18 * Definition:
19 * ===========
20 *
21 * SUBROUTINE STGSY2( TRANS, IJOB, M, N, A, LDA, B, LDB, C, LDC, D,
22 * LDD, E, LDE, F, LDF, SCALE, RDSUM, RDSCAL,
23 * IWORK, PQ, INFO )
24 *
25 * .. Scalar Arguments ..
26 * CHARACTER TRANS
27 * INTEGER IJOB, INFO, LDA, LDB, LDC, LDD, LDE, LDF, M, N,
28 * $ PQ
29 * REAL RDSCAL, RDSUM, SCALE
30 * ..
31 * .. Array Arguments ..
32 * INTEGER IWORK( * )
33 * REAL A( LDA, * ), B( LDB, * ), C( LDC, * ),
34 * $ D( LDD, * ), E( LDE, * ), F( LDF, * )
35 * ..
36 *
37 *
38 *> \par Purpose:
39 * =============
40 *>
41 *> \verbatim
42 *>
43 *> STGSY2 solves the generalized Sylvester equation:
44 *>
45 *> A * R - L * B = scale * C (1)
46 *> D * R - L * E = scale * F,
47 *>
48 *> using Level 1 and 2 BLAS. where R and L are unknown M-by-N matrices,
49 *> (A, D), (B, E) and (C, F) are given matrix pairs of size M-by-M,
50 *> N-by-N and M-by-N, respectively, with real entries. (A, D) and (B, E)
51 *> must be in generalized Schur canonical form, i.e. A, B are upper
52 *> quasi triangular and D, E are upper triangular. The solution (R, L)
53 *> overwrites (C, F). 0 <= SCALE <= 1 is an output scaling factor
54 *> chosen to avoid overflow.
55 *>
56 *> In matrix notation solving equation (1) corresponds to solve
57 *> Z*x = scale*b, where Z is defined as
58 *>
59 *> Z = [ kron(In, A) -kron(B**T, Im) ] (2)
60 *> [ kron(In, D) -kron(E**T, Im) ],
61 *>
62 *> Ik is the identity matrix of size k and X**T is the transpose of X.
63 *> kron(X, Y) is the Kronecker product between the matrices X and Y.
64 *> In the process of solving (1), we solve a number of such systems
65 *> where Dim(In), Dim(In) = 1 or 2.
66 *>
67 *> If TRANS = 'T', solve the transposed system Z**T*y = scale*b for y,
68 *> which is equivalent to solve for R and L in
69 *>
70 *> A**T * R + D**T * L = scale * C (3)
71 *> R * B**T + L * E**T = scale * -F
72 *>
73 *> This case is used to compute an estimate of Dif[(A, D), (B, E)] =
74 *> sigma_min(Z) using reverse communicaton with SLACON.
75 *>
76 *> STGSY2 also (IJOB >= 1) contributes to the computation in STGSYL
77 *> of an upper bound on the separation between to matrix pairs. Then
78 *> the input (A, D), (B, E) are sub-pencils of the matrix pair in
79 *> STGSYL. See STGSYL for details.
80 *> \endverbatim
81 *
82 * Arguments:
83 * ==========
84 *
85 *> \param[in] TRANS
86 *> \verbatim
87 *> TRANS is CHARACTER*1
88 *> = 'N', solve the generalized Sylvester equation (1).
89 *> = 'T': solve the 'transposed' system (3).
90 *> \endverbatim
91 *>
92 *> \param[in] IJOB
93 *> \verbatim
94 *> IJOB is INTEGER
95 *> Specifies what kind of functionality to be performed.
96 *> = 0: solve (1) only.
97 *> = 1: A contribution from this subsystem to a Frobenius
98 *> norm-based estimate of the separation between two matrix
99 *> pairs is computed. (look ahead strategy is used).
100 *> = 2: A contribution from this subsystem to a Frobenius
101 *> norm-based estimate of the separation between two matrix
102 *> pairs is computed. (SGECON on sub-systems is used.)
103 *> Not referenced if TRANS = 'T'.
104 *> \endverbatim
105 *>
106 *> \param[in] M
107 *> \verbatim
108 *> M is INTEGER
109 *> On entry, M specifies the order of A and D, and the row
110 *> dimension of C, F, R and L.
111 *> \endverbatim
112 *>
113 *> \param[in] N
114 *> \verbatim
115 *> N is INTEGER
116 *> On entry, N specifies the order of B and E, and the column
117 *> dimension of C, F, R and L.
118 *> \endverbatim
119 *>
120 *> \param[in] A
121 *> \verbatim
122 *> A is REAL array, dimension (LDA, M)
123 *> On entry, A contains an upper quasi triangular matrix.
124 *> \endverbatim
125 *>
126 *> \param[in] LDA
127 *> \verbatim
128 *> LDA is INTEGER
129 *> The leading dimension of the matrix A. LDA >= max(1, M).
130 *> \endverbatim
131 *>
132 *> \param[in] B
133 *> \verbatim
134 *> B is REAL array, dimension (LDB, N)
135 *> On entry, B contains an upper quasi triangular matrix.
136 *> \endverbatim
137 *>
138 *> \param[in] LDB
139 *> \verbatim
140 *> LDB is INTEGER
141 *> The leading dimension of the matrix B. LDB >= max(1, N).
142 *> \endverbatim
143 *>
144 *> \param[in,out] C
145 *> \verbatim
146 *> C is REAL array, dimension (LDC, N)
147 *> On entry, C contains the right-hand-side of the first matrix
148 *> equation in (1).
149 *> On exit, if IJOB = 0, C has been overwritten by the
150 *> solution R.
151 *> \endverbatim
152 *>
153 *> \param[in] LDC
154 *> \verbatim
155 *> LDC is INTEGER
156 *> The leading dimension of the matrix C. LDC >= max(1, M).
157 *> \endverbatim
158 *>
159 *> \param[in] D
160 *> \verbatim
161 *> D is REAL array, dimension (LDD, M)
162 *> On entry, D contains an upper triangular matrix.
163 *> \endverbatim
164 *>
165 *> \param[in] LDD
166 *> \verbatim
167 *> LDD is INTEGER
168 *> The leading dimension of the matrix D. LDD >= max(1, M).
169 *> \endverbatim
170 *>
171 *> \param[in] E
172 *> \verbatim
173 *> E is REAL array, dimension (LDE, N)
174 *> On entry, E contains an upper triangular matrix.
175 *> \endverbatim
176 *>
177 *> \param[in] LDE
178 *> \verbatim
179 *> LDE is INTEGER
180 *> The leading dimension of the matrix E. LDE >= max(1, N).
181 *> \endverbatim
182 *>
183 *> \param[in,out] F
184 *> \verbatim
185 *> F is REAL array, dimension (LDF, N)
186 *> On entry, F contains the right-hand-side of the second matrix
187 *> equation in (1).
188 *> On exit, if IJOB = 0, F has been overwritten by the
189 *> solution L.
190 *> \endverbatim
191 *>
192 *> \param[in] LDF
193 *> \verbatim
194 *> LDF is INTEGER
195 *> The leading dimension of the matrix F. LDF >= max(1, M).
196 *> \endverbatim
197 *>
198 *> \param[out] SCALE
199 *> \verbatim
200 *> SCALE is REAL
201 *> On exit, 0 <= SCALE <= 1. If 0 < SCALE < 1, the solutions
202 *> R and L (C and F on entry) will hold the solutions to a
203 *> slightly perturbed system but the input matrices A, B, D and
204 *> E have not been changed. If SCALE = 0, R and L will hold the
205 *> solutions to the homogeneous system with C = F = 0. Normally,
206 *> SCALE = 1.
207 *> \endverbatim
208 *>
209 *> \param[in,out] RDSUM
210 *> \verbatim
211 *> RDSUM is REAL
212 *> On entry, the sum of squares of computed contributions to
213 *> the Dif-estimate under computation by STGSYL, where the
214 *> scaling factor RDSCAL (see below) has been factored out.
215 *> On exit, the corresponding sum of squares updated with the
216 *> contributions from the current sub-system.
217 *> If TRANS = 'T' RDSUM is not touched.
218 *> NOTE: RDSUM only makes sense when STGSY2 is called by STGSYL.
219 *> \endverbatim
220 *>
221 *> \param[in,out] RDSCAL
222 *> \verbatim
223 *> RDSCAL is REAL
224 *> On entry, scaling factor used to prevent overflow in RDSUM.
225 *> On exit, RDSCAL is updated w.r.t. the current contributions
226 *> in RDSUM.
227 *> If TRANS = 'T', RDSCAL is not touched.
228 *> NOTE: RDSCAL only makes sense when STGSY2 is called by
229 *> STGSYL.
230 *> \endverbatim
231 *>
232 *> \param[out] IWORK
233 *> \verbatim
234 *> IWORK is INTEGER array, dimension (M+N+2)
235 *> \endverbatim
236 *>
237 *> \param[out] PQ
238 *> \verbatim
239 *> PQ is INTEGER
240 *> On exit, the number of subsystems (of size 2-by-2, 4-by-4 and
241 *> 8-by-8) solved by this routine.
242 *> \endverbatim
243 *>
244 *> \param[out] INFO
245 *> \verbatim
246 *> INFO is INTEGER
247 *> On exit, if INFO is set to
248 *> =0: Successful exit
249 *> <0: If INFO = -i, the i-th argument had an illegal value.
250 *> >0: The matrix pairs (A, D) and (B, E) have common or very
251 *> close eigenvalues.
252 *> \endverbatim
253 *
254 * Authors:
255 * ========
256 *
257 *> \author Univ. of Tennessee
258 *> \author Univ. of California Berkeley
259 *> \author Univ. of Colorado Denver
260 *> \author NAG Ltd.
261 *
262 *> \date September 2012
263 *
264 *> \ingroup realSYauxiliary
265 *
266 *> \par Contributors:
267 * ==================
268 *>
269 *> Bo Kagstrom and Peter Poromaa, Department of Computing Science,
270 *> Umea University, S-901 87 Umea, Sweden.
271 *
272 * =====================================================================
273  SUBROUTINE stgsy2( TRANS, IJOB, M, N, A, LDA, B, LDB, C, LDC, D,
274  $ ldd, e, lde, f, ldf, scale, rdsum, rdscal,
275  $ iwork, pq, info )
276 *
277 * -- LAPACK auxiliary routine (version 3.4.2) --
278 * -- LAPACK is a software package provided by Univ. of Tennessee, --
279 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
280 * September 2012
281 *
282 * .. Scalar Arguments ..
283  CHARACTER trans
284  INTEGER ijob, info, lda, ldb, ldc, ldd, lde, ldf, m, n,
285  $ pq
286  REAL rdscal, rdsum, scale
287 * ..
288 * .. Array Arguments ..
289  INTEGER iwork( * )
290  REAL a( lda, * ), b( ldb, * ), c( ldc, * ),
291  $ d( ldd, * ), e( lde, * ), f( ldf, * )
292 * ..
293 *
294 * =====================================================================
295 * Replaced various illegal calls to SCOPY by calls to SLASET.
296 * Sven Hammarling, 27/5/02.
297 *
298 * .. Parameters ..
299  INTEGER ldz
300  parameter( ldz = 8 )
301  REAL zero, one
302  parameter( zero = 0.0e+0, one = 1.0e+0 )
303 * ..
304 * .. Local Scalars ..
305  LOGICAL notran
306  INTEGER i, ie, ierr, ii, is, isp1, j, je, jj, js, jsp1,
307  $ k, mb, nb, p, q, zdim
308  REAL alpha, scaloc
309 * ..
310 * .. Local Arrays ..
311  INTEGER ipiv( ldz ), jpiv( ldz )
312  REAL rhs( ldz ), z( ldz, ldz )
313 * ..
314 * .. External Functions ..
315  LOGICAL lsame
316  EXTERNAL lsame
317 * ..
318 * .. External Subroutines ..
319  EXTERNAL saxpy, scopy, sgemm, sgemv, sger, sgesc2,
321 * ..
322 * .. Intrinsic Functions ..
323  INTRINSIC max
324 * ..
325 * .. Executable Statements ..
326 *
327 * Decode and test input parameters
328 *
329  info = 0
330  ierr = 0
331  notran = lsame( trans, 'N' )
332  IF( .NOT.notran .AND. .NOT.lsame( trans, 'T' ) ) THEN
333  info = -1
334  ELSE IF( notran ) THEN
335  IF( ( ijob.LT.0 ) .OR. ( ijob.GT.2 ) ) THEN
336  info = -2
337  END IF
338  END IF
339  IF( info.EQ.0 ) THEN
340  IF( m.LE.0 ) THEN
341  info = -3
342  ELSE IF( n.LE.0 ) THEN
343  info = -4
344  ELSE IF( lda.LT.max( 1, m ) ) THEN
345  info = -5
346  ELSE IF( ldb.LT.max( 1, n ) ) THEN
347  info = -8
348  ELSE IF( ldc.LT.max( 1, m ) ) THEN
349  info = -10
350  ELSE IF( ldd.LT.max( 1, m ) ) THEN
351  info = -12
352  ELSE IF( lde.LT.max( 1, n ) ) THEN
353  info = -14
354  ELSE IF( ldf.LT.max( 1, m ) ) THEN
355  info = -16
356  END IF
357  END IF
358  IF( info.NE.0 ) THEN
359  CALL xerbla( 'STGSY2', -info )
360  RETURN
361  END IF
362 *
363 * Determine block structure of A
364 *
365  pq = 0
366  p = 0
367  i = 1
368  10 CONTINUE
369  IF( i.GT.m )
370  $ go to 20
371  p = p + 1
372  iwork( p ) = i
373  IF( i.EQ.m )
374  $ go to 20
375  IF( a( i+1, i ).NE.zero ) THEN
376  i = i + 2
377  ELSE
378  i = i + 1
379  END IF
380  go to 10
381  20 CONTINUE
382  iwork( p+1 ) = m + 1
383 *
384 * Determine block structure of B
385 *
386  q = p + 1
387  j = 1
388  30 CONTINUE
389  IF( j.GT.n )
390  $ go to 40
391  q = q + 1
392  iwork( q ) = j
393  IF( j.EQ.n )
394  $ go to 40
395  IF( b( j+1, j ).NE.zero ) THEN
396  j = j + 2
397  ELSE
398  j = j + 1
399  END IF
400  go to 30
401  40 CONTINUE
402  iwork( q+1 ) = n + 1
403  pq = p*( q-p-1 )
404 *
405  IF( notran ) THEN
406 *
407 * Solve (I, J) - subsystem
408 * A(I, I) * R(I, J) - L(I, J) * B(J, J) = C(I, J)
409 * D(I, I) * R(I, J) - L(I, J) * E(J, J) = F(I, J)
410 * for I = P, P - 1, ..., 1; J = 1, 2, ..., Q
411 *
412  scale = one
413  scaloc = one
414  DO 120 j = p + 2, q
415  js = iwork( j )
416  jsp1 = js + 1
417  je = iwork( j+1 ) - 1
418  nb = je - js + 1
419  DO 110 i = p, 1, -1
420 *
421  is = iwork( i )
422  isp1 = is + 1
423  ie = iwork( i+1 ) - 1
424  mb = ie - is + 1
425  zdim = mb*nb*2
426 *
427  IF( ( mb.EQ.1 ) .AND. ( nb.EQ.1 ) ) THEN
428 *
429 * Build a 2-by-2 system Z * x = RHS
430 *
431  z( 1, 1 ) = a( is, is )
432  z( 2, 1 ) = d( is, is )
433  z( 1, 2 ) = -b( js, js )
434  z( 2, 2 ) = -e( js, js )
435 *
436 * Set up right hand side(s)
437 *
438  rhs( 1 ) = c( is, js )
439  rhs( 2 ) = f( is, js )
440 *
441 * Solve Z * x = RHS
442 *
443  CALL sgetc2( zdim, z, ldz, ipiv, jpiv, ierr )
444  IF( ierr.GT.0 )
445  $ info = ierr
446 *
447  IF( ijob.EQ.0 ) THEN
448  CALL sgesc2( zdim, z, ldz, rhs, ipiv, jpiv,
449  $ scaloc )
450  IF( scaloc.NE.one ) THEN
451  DO 50 k = 1, n
452  CALL sscal( m, scaloc, c( 1, k ), 1 )
453  CALL sscal( m, scaloc, f( 1, k ), 1 )
454  50 CONTINUE
455  scale = scale*scaloc
456  END IF
457  ELSE
458  CALL slatdf( ijob, zdim, z, ldz, rhs, rdsum,
459  $ rdscal, ipiv, jpiv )
460  END IF
461 *
462 * Unpack solution vector(s)
463 *
464  c( is, js ) = rhs( 1 )
465  f( is, js ) = rhs( 2 )
466 *
467 * Substitute R(I, J) and L(I, J) into remaining
468 * equation.
469 *
470  IF( i.GT.1 ) THEN
471  alpha = -rhs( 1 )
472  CALL saxpy( is-1, alpha, a( 1, is ), 1, c( 1, js ),
473  $ 1 )
474  CALL saxpy( is-1, alpha, d( 1, is ), 1, f( 1, js ),
475  $ 1 )
476  END IF
477  IF( j.LT.q ) THEN
478  CALL saxpy( n-je, rhs( 2 ), b( js, je+1 ), ldb,
479  $ c( is, je+1 ), ldc )
480  CALL saxpy( n-je, rhs( 2 ), e( js, je+1 ), lde,
481  $ f( is, je+1 ), ldf )
482  END IF
483 *
484  ELSE IF( ( mb.EQ.1 ) .AND. ( nb.EQ.2 ) ) THEN
485 *
486 * Build a 4-by-4 system Z * x = RHS
487 *
488  z( 1, 1 ) = a( is, is )
489  z( 2, 1 ) = zero
490  z( 3, 1 ) = d( is, is )
491  z( 4, 1 ) = zero
492 *
493  z( 1, 2 ) = zero
494  z( 2, 2 ) = a( is, is )
495  z( 3, 2 ) = zero
496  z( 4, 2 ) = d( is, is )
497 *
498  z( 1, 3 ) = -b( js, js )
499  z( 2, 3 ) = -b( js, jsp1 )
500  z( 3, 3 ) = -e( js, js )
501  z( 4, 3 ) = -e( js, jsp1 )
502 *
503  z( 1, 4 ) = -b( jsp1, js )
504  z( 2, 4 ) = -b( jsp1, jsp1 )
505  z( 3, 4 ) = zero
506  z( 4, 4 ) = -e( jsp1, jsp1 )
507 *
508 * Set up right hand side(s)
509 *
510  rhs( 1 ) = c( is, js )
511  rhs( 2 ) = c( is, jsp1 )
512  rhs( 3 ) = f( is, js )
513  rhs( 4 ) = f( is, jsp1 )
514 *
515 * Solve Z * x = RHS
516 *
517  CALL sgetc2( zdim, z, ldz, ipiv, jpiv, ierr )
518  IF( ierr.GT.0 )
519  $ info = ierr
520 *
521  IF( ijob.EQ.0 ) THEN
522  CALL sgesc2( zdim, z, ldz, rhs, ipiv, jpiv,
523  $ scaloc )
524  IF( scaloc.NE.one ) THEN
525  DO 60 k = 1, n
526  CALL sscal( m, scaloc, c( 1, k ), 1 )
527  CALL sscal( m, scaloc, f( 1, k ), 1 )
528  60 CONTINUE
529  scale = scale*scaloc
530  END IF
531  ELSE
532  CALL slatdf( ijob, zdim, z, ldz, rhs, rdsum,
533  $ rdscal, ipiv, jpiv )
534  END IF
535 *
536 * Unpack solution vector(s)
537 *
538  c( is, js ) = rhs( 1 )
539  c( is, jsp1 ) = rhs( 2 )
540  f( is, js ) = rhs( 3 )
541  f( is, jsp1 ) = rhs( 4 )
542 *
543 * Substitute R(I, J) and L(I, J) into remaining
544 * equation.
545 *
546  IF( i.GT.1 ) THEN
547  CALL sger( is-1, nb, -one, a( 1, is ), 1, rhs( 1 ),
548  $ 1, c( 1, js ), ldc )
549  CALL sger( is-1, nb, -one, d( 1, is ), 1, rhs( 1 ),
550  $ 1, f( 1, js ), ldf )
551  END IF
552  IF( j.LT.q ) THEN
553  CALL saxpy( n-je, rhs( 3 ), b( js, je+1 ), ldb,
554  $ c( is, je+1 ), ldc )
555  CALL saxpy( n-je, rhs( 3 ), e( js, je+1 ), lde,
556  $ f( is, je+1 ), ldf )
557  CALL saxpy( n-je, rhs( 4 ), b( jsp1, je+1 ), ldb,
558  $ c( is, je+1 ), ldc )
559  CALL saxpy( n-je, rhs( 4 ), e( jsp1, je+1 ), lde,
560  $ f( is, je+1 ), ldf )
561  END IF
562 *
563  ELSE IF( ( mb.EQ.2 ) .AND. ( nb.EQ.1 ) ) THEN
564 *
565 * Build a 4-by-4 system Z * x = RHS
566 *
567  z( 1, 1 ) = a( is, is )
568  z( 2, 1 ) = a( isp1, is )
569  z( 3, 1 ) = d( is, is )
570  z( 4, 1 ) = zero
571 *
572  z( 1, 2 ) = a( is, isp1 )
573  z( 2, 2 ) = a( isp1, isp1 )
574  z( 3, 2 ) = d( is, isp1 )
575  z( 4, 2 ) = d( isp1, isp1 )
576 *
577  z( 1, 3 ) = -b( js, js )
578  z( 2, 3 ) = zero
579  z( 3, 3 ) = -e( js, js )
580  z( 4, 3 ) = zero
581 *
582  z( 1, 4 ) = zero
583  z( 2, 4 ) = -b( js, js )
584  z( 3, 4 ) = zero
585  z( 4, 4 ) = -e( js, js )
586 *
587 * Set up right hand side(s)
588 *
589  rhs( 1 ) = c( is, js )
590  rhs( 2 ) = c( isp1, js )
591  rhs( 3 ) = f( is, js )
592  rhs( 4 ) = f( isp1, js )
593 *
594 * Solve Z * x = RHS
595 *
596  CALL sgetc2( zdim, z, ldz, ipiv, jpiv, ierr )
597  IF( ierr.GT.0 )
598  $ info = ierr
599  IF( ijob.EQ.0 ) THEN
600  CALL sgesc2( zdim, z, ldz, rhs, ipiv, jpiv,
601  $ scaloc )
602  IF( scaloc.NE.one ) THEN
603  DO 70 k = 1, n
604  CALL sscal( m, scaloc, c( 1, k ), 1 )
605  CALL sscal( m, scaloc, f( 1, k ), 1 )
606  70 CONTINUE
607  scale = scale*scaloc
608  END IF
609  ELSE
610  CALL slatdf( ijob, zdim, z, ldz, rhs, rdsum,
611  $ rdscal, ipiv, jpiv )
612  END IF
613 *
614 * Unpack solution vector(s)
615 *
616  c( is, js ) = rhs( 1 )
617  c( isp1, js ) = rhs( 2 )
618  f( is, js ) = rhs( 3 )
619  f( isp1, js ) = rhs( 4 )
620 *
621 * Substitute R(I, J) and L(I, J) into remaining
622 * equation.
623 *
624  IF( i.GT.1 ) THEN
625  CALL sgemv( 'N', is-1, mb, -one, a( 1, is ), lda,
626  $ rhs( 1 ), 1, one, c( 1, js ), 1 )
627  CALL sgemv( 'N', is-1, mb, -one, d( 1, is ), ldd,
628  $ rhs( 1 ), 1, one, f( 1, js ), 1 )
629  END IF
630  IF( j.LT.q ) THEN
631  CALL sger( mb, n-je, one, rhs( 3 ), 1,
632  $ b( js, je+1 ), ldb, c( is, je+1 ), ldc )
633  CALL sger( mb, n-je, one, rhs( 3 ), 1,
634  $ e( js, je+1 ), lde, f( is, je+1 ), ldf )
635  END IF
636 *
637  ELSE IF( ( mb.EQ.2 ) .AND. ( nb.EQ.2 ) ) THEN
638 *
639 * Build an 8-by-8 system Z * x = RHS
640 *
641  CALL slaset( 'F', ldz, ldz, zero, zero, z, ldz )
642 *
643  z( 1, 1 ) = a( is, is )
644  z( 2, 1 ) = a( isp1, is )
645  z( 5, 1 ) = d( is, is )
646 *
647  z( 1, 2 ) = a( is, isp1 )
648  z( 2, 2 ) = a( isp1, isp1 )
649  z( 5, 2 ) = d( is, isp1 )
650  z( 6, 2 ) = d( isp1, isp1 )
651 *
652  z( 3, 3 ) = a( is, is )
653  z( 4, 3 ) = a( isp1, is )
654  z( 7, 3 ) = d( is, is )
655 *
656  z( 3, 4 ) = a( is, isp1 )
657  z( 4, 4 ) = a( isp1, isp1 )
658  z( 7, 4 ) = d( is, isp1 )
659  z( 8, 4 ) = d( isp1, isp1 )
660 *
661  z( 1, 5 ) = -b( js, js )
662  z( 3, 5 ) = -b( js, jsp1 )
663  z( 5, 5 ) = -e( js, js )
664  z( 7, 5 ) = -e( js, jsp1 )
665 *
666  z( 2, 6 ) = -b( js, js )
667  z( 4, 6 ) = -b( js, jsp1 )
668  z( 6, 6 ) = -e( js, js )
669  z( 8, 6 ) = -e( js, jsp1 )
670 *
671  z( 1, 7 ) = -b( jsp1, js )
672  z( 3, 7 ) = -b( jsp1, jsp1 )
673  z( 7, 7 ) = -e( jsp1, jsp1 )
674 *
675  z( 2, 8 ) = -b( jsp1, js )
676  z( 4, 8 ) = -b( jsp1, jsp1 )
677  z( 8, 8 ) = -e( jsp1, jsp1 )
678 *
679 * Set up right hand side(s)
680 *
681  k = 1
682  ii = mb*nb + 1
683  DO 80 jj = 0, nb - 1
684  CALL scopy( mb, c( is, js+jj ), 1, rhs( k ), 1 )
685  CALL scopy( mb, f( is, js+jj ), 1, rhs( ii ), 1 )
686  k = k + mb
687  ii = ii + mb
688  80 CONTINUE
689 *
690 * Solve Z * x = RHS
691 *
692  CALL sgetc2( zdim, z, ldz, ipiv, jpiv, ierr )
693  IF( ierr.GT.0 )
694  $ info = ierr
695  IF( ijob.EQ.0 ) THEN
696  CALL sgesc2( zdim, z, ldz, rhs, ipiv, jpiv,
697  $ scaloc )
698  IF( scaloc.NE.one ) THEN
699  DO 90 k = 1, n
700  CALL sscal( m, scaloc, c( 1, k ), 1 )
701  CALL sscal( m, scaloc, f( 1, k ), 1 )
702  90 CONTINUE
703  scale = scale*scaloc
704  END IF
705  ELSE
706  CALL slatdf( ijob, zdim, z, ldz, rhs, rdsum,
707  $ rdscal, ipiv, jpiv )
708  END IF
709 *
710 * Unpack solution vector(s)
711 *
712  k = 1
713  ii = mb*nb + 1
714  DO 100 jj = 0, nb - 1
715  CALL scopy( mb, rhs( k ), 1, c( is, js+jj ), 1 )
716  CALL scopy( mb, rhs( ii ), 1, f( is, js+jj ), 1 )
717  k = k + mb
718  ii = ii + mb
719  100 CONTINUE
720 *
721 * Substitute R(I, J) and L(I, J) into remaining
722 * equation.
723 *
724  IF( i.GT.1 ) THEN
725  CALL sgemm( 'N', 'N', is-1, nb, mb, -one,
726  $ a( 1, is ), lda, rhs( 1 ), mb, one,
727  $ c( 1, js ), ldc )
728  CALL sgemm( 'N', 'N', is-1, nb, mb, -one,
729  $ d( 1, is ), ldd, rhs( 1 ), mb, one,
730  $ f( 1, js ), ldf )
731  END IF
732  IF( j.LT.q ) THEN
733  k = mb*nb + 1
734  CALL sgemm( 'N', 'N', mb, n-je, nb, one, rhs( k ),
735  $ mb, b( js, je+1 ), ldb, one,
736  $ c( is, je+1 ), ldc )
737  CALL sgemm( 'N', 'N', mb, n-je, nb, one, rhs( k ),
738  $ mb, e( js, je+1 ), lde, one,
739  $ f( is, je+1 ), ldf )
740  END IF
741 *
742  END IF
743 *
744  110 CONTINUE
745  120 CONTINUE
746  ELSE
747 *
748 * Solve (I, J) - subsystem
749 * A(I, I)**T * R(I, J) + D(I, I)**T * L(J, J) = C(I, J)
750 * R(I, I) * B(J, J) + L(I, J) * E(J, J) = -F(I, J)
751 * for I = 1, 2, ..., P, J = Q, Q - 1, ..., 1
752 *
753  scale = one
754  scaloc = one
755  DO 200 i = 1, p
756 *
757  is = iwork( i )
758  isp1 = is + 1
759  ie = iwork( i+1 ) - 1
760  mb = ie - is + 1
761  DO 190 j = q, p + 2, -1
762 *
763  js = iwork( j )
764  jsp1 = js + 1
765  je = iwork( j+1 ) - 1
766  nb = je - js + 1
767  zdim = mb*nb*2
768  IF( ( mb.EQ.1 ) .AND. ( nb.EQ.1 ) ) THEN
769 *
770 * Build a 2-by-2 system Z**T * x = RHS
771 *
772  z( 1, 1 ) = a( is, is )
773  z( 2, 1 ) = -b( js, js )
774  z( 1, 2 ) = d( is, is )
775  z( 2, 2 ) = -e( js, js )
776 *
777 * Set up right hand side(s)
778 *
779  rhs( 1 ) = c( is, js )
780  rhs( 2 ) = f( is, js )
781 *
782 * Solve Z**T * x = RHS
783 *
784  CALL sgetc2( zdim, z, ldz, ipiv, jpiv, ierr )
785  IF( ierr.GT.0 )
786  $ info = ierr
787 *
788  CALL sgesc2( zdim, z, ldz, rhs, ipiv, jpiv, scaloc )
789  IF( scaloc.NE.one ) THEN
790  DO 130 k = 1, n
791  CALL sscal( m, scaloc, c( 1, k ), 1 )
792  CALL sscal( m, scaloc, f( 1, k ), 1 )
793  130 CONTINUE
794  scale = scale*scaloc
795  END IF
796 *
797 * Unpack solution vector(s)
798 *
799  c( is, js ) = rhs( 1 )
800  f( is, js ) = rhs( 2 )
801 *
802 * Substitute R(I, J) and L(I, J) into remaining
803 * equation.
804 *
805  IF( j.GT.p+2 ) THEN
806  alpha = rhs( 1 )
807  CALL saxpy( js-1, alpha, b( 1, js ), 1, f( is, 1 ),
808  $ ldf )
809  alpha = rhs( 2 )
810  CALL saxpy( js-1, alpha, e( 1, js ), 1, f( is, 1 ),
811  $ ldf )
812  END IF
813  IF( i.LT.p ) THEN
814  alpha = -rhs( 1 )
815  CALL saxpy( m-ie, alpha, a( is, ie+1 ), lda,
816  $ c( ie+1, js ), 1 )
817  alpha = -rhs( 2 )
818  CALL saxpy( m-ie, alpha, d( is, ie+1 ), ldd,
819  $ c( ie+1, js ), 1 )
820  END IF
821 *
822  ELSE IF( ( mb.EQ.1 ) .AND. ( nb.EQ.2 ) ) THEN
823 *
824 * Build a 4-by-4 system Z**T * x = RHS
825 *
826  z( 1, 1 ) = a( is, is )
827  z( 2, 1 ) = zero
828  z( 3, 1 ) = -b( js, js )
829  z( 4, 1 ) = -b( jsp1, js )
830 *
831  z( 1, 2 ) = zero
832  z( 2, 2 ) = a( is, is )
833  z( 3, 2 ) = -b( js, jsp1 )
834  z( 4, 2 ) = -b( jsp1, jsp1 )
835 *
836  z( 1, 3 ) = d( is, is )
837  z( 2, 3 ) = zero
838  z( 3, 3 ) = -e( js, js )
839  z( 4, 3 ) = zero
840 *
841  z( 1, 4 ) = zero
842  z( 2, 4 ) = d( is, is )
843  z( 3, 4 ) = -e( js, jsp1 )
844  z( 4, 4 ) = -e( jsp1, jsp1 )
845 *
846 * Set up right hand side(s)
847 *
848  rhs( 1 ) = c( is, js )
849  rhs( 2 ) = c( is, jsp1 )
850  rhs( 3 ) = f( is, js )
851  rhs( 4 ) = f( is, jsp1 )
852 *
853 * Solve Z**T * x = RHS
854 *
855  CALL sgetc2( zdim, z, ldz, ipiv, jpiv, ierr )
856  IF( ierr.GT.0 )
857  $ info = ierr
858  CALL sgesc2( zdim, z, ldz, rhs, ipiv, jpiv, scaloc )
859  IF( scaloc.NE.one ) THEN
860  DO 140 k = 1, n
861  CALL sscal( m, scaloc, c( 1, k ), 1 )
862  CALL sscal( m, scaloc, f( 1, k ), 1 )
863  140 CONTINUE
864  scale = scale*scaloc
865  END IF
866 *
867 * Unpack solution vector(s)
868 *
869  c( is, js ) = rhs( 1 )
870  c( is, jsp1 ) = rhs( 2 )
871  f( is, js ) = rhs( 3 )
872  f( is, jsp1 ) = rhs( 4 )
873 *
874 * Substitute R(I, J) and L(I, J) into remaining
875 * equation.
876 *
877  IF( j.GT.p+2 ) THEN
878  CALL saxpy( js-1, rhs( 1 ), b( 1, js ), 1,
879  $ f( is, 1 ), ldf )
880  CALL saxpy( js-1, rhs( 2 ), b( 1, jsp1 ), 1,
881  $ f( is, 1 ), ldf )
882  CALL saxpy( js-1, rhs( 3 ), e( 1, js ), 1,
883  $ f( is, 1 ), ldf )
884  CALL saxpy( js-1, rhs( 4 ), e( 1, jsp1 ), 1,
885  $ f( is, 1 ), ldf )
886  END IF
887  IF( i.LT.p ) THEN
888  CALL sger( m-ie, nb, -one, a( is, ie+1 ), lda,
889  $ rhs( 1 ), 1, c( ie+1, js ), ldc )
890  CALL sger( m-ie, nb, -one, d( is, ie+1 ), ldd,
891  $ rhs( 3 ), 1, c( ie+1, js ), ldc )
892  END IF
893 *
894  ELSE IF( ( mb.EQ.2 ) .AND. ( nb.EQ.1 ) ) THEN
895 *
896 * Build a 4-by-4 system Z**T * x = RHS
897 *
898  z( 1, 1 ) = a( is, is )
899  z( 2, 1 ) = a( is, isp1 )
900  z( 3, 1 ) = -b( js, js )
901  z( 4, 1 ) = zero
902 *
903  z( 1, 2 ) = a( isp1, is )
904  z( 2, 2 ) = a( isp1, isp1 )
905  z( 3, 2 ) = zero
906  z( 4, 2 ) = -b( js, js )
907 *
908  z( 1, 3 ) = d( is, is )
909  z( 2, 3 ) = d( is, isp1 )
910  z( 3, 3 ) = -e( js, js )
911  z( 4, 3 ) = zero
912 *
913  z( 1, 4 ) = zero
914  z( 2, 4 ) = d( isp1, isp1 )
915  z( 3, 4 ) = zero
916  z( 4, 4 ) = -e( js, js )
917 *
918 * Set up right hand side(s)
919 *
920  rhs( 1 ) = c( is, js )
921  rhs( 2 ) = c( isp1, js )
922  rhs( 3 ) = f( is, js )
923  rhs( 4 ) = f( isp1, js )
924 *
925 * Solve Z**T * x = RHS
926 *
927  CALL sgetc2( zdim, z, ldz, ipiv, jpiv, ierr )
928  IF( ierr.GT.0 )
929  $ info = ierr
930 *
931  CALL sgesc2( zdim, z, ldz, rhs, ipiv, jpiv, scaloc )
932  IF( scaloc.NE.one ) THEN
933  DO 150 k = 1, n
934  CALL sscal( m, scaloc, c( 1, k ), 1 )
935  CALL sscal( m, scaloc, f( 1, k ), 1 )
936  150 CONTINUE
937  scale = scale*scaloc
938  END IF
939 *
940 * Unpack solution vector(s)
941 *
942  c( is, js ) = rhs( 1 )
943  c( isp1, js ) = rhs( 2 )
944  f( is, js ) = rhs( 3 )
945  f( isp1, js ) = rhs( 4 )
946 *
947 * Substitute R(I, J) and L(I, J) into remaining
948 * equation.
949 *
950  IF( j.GT.p+2 ) THEN
951  CALL sger( mb, js-1, one, rhs( 1 ), 1, b( 1, js ),
952  $ 1, f( is, 1 ), ldf )
953  CALL sger( mb, js-1, one, rhs( 3 ), 1, e( 1, js ),
954  $ 1, f( is, 1 ), ldf )
955  END IF
956  IF( i.LT.p ) THEN
957  CALL sgemv( 'T', mb, m-ie, -one, a( is, ie+1 ),
958  $ lda, rhs( 1 ), 1, one, c( ie+1, js ),
959  $ 1 )
960  CALL sgemv( 'T', mb, m-ie, -one, d( is, ie+1 ),
961  $ ldd, rhs( 3 ), 1, one, c( ie+1, js ),
962  $ 1 )
963  END IF
964 *
965  ELSE IF( ( mb.EQ.2 ) .AND. ( nb.EQ.2 ) ) THEN
966 *
967 * Build an 8-by-8 system Z**T * x = RHS
968 *
969  CALL slaset( 'F', ldz, ldz, zero, zero, z, ldz )
970 *
971  z( 1, 1 ) = a( is, is )
972  z( 2, 1 ) = a( is, isp1 )
973  z( 5, 1 ) = -b( js, js )
974  z( 7, 1 ) = -b( jsp1, js )
975 *
976  z( 1, 2 ) = a( isp1, is )
977  z( 2, 2 ) = a( isp1, isp1 )
978  z( 6, 2 ) = -b( js, js )
979  z( 8, 2 ) = -b( jsp1, js )
980 *
981  z( 3, 3 ) = a( is, is )
982  z( 4, 3 ) = a( is, isp1 )
983  z( 5, 3 ) = -b( js, jsp1 )
984  z( 7, 3 ) = -b( jsp1, jsp1 )
985 *
986  z( 3, 4 ) = a( isp1, is )
987  z( 4, 4 ) = a( isp1, isp1 )
988  z( 6, 4 ) = -b( js, jsp1 )
989  z( 8, 4 ) = -b( jsp1, jsp1 )
990 *
991  z( 1, 5 ) = d( is, is )
992  z( 2, 5 ) = d( is, isp1 )
993  z( 5, 5 ) = -e( js, js )
994 *
995  z( 2, 6 ) = d( isp1, isp1 )
996  z( 6, 6 ) = -e( js, js )
997 *
998  z( 3, 7 ) = d( is, is )
999  z( 4, 7 ) = d( is, isp1 )
1000  z( 5, 7 ) = -e( js, jsp1 )
1001  z( 7, 7 ) = -e( jsp1, jsp1 )
1002 *
1003  z( 4, 8 ) = d( isp1, isp1 )
1004  z( 6, 8 ) = -e( js, jsp1 )
1005  z( 8, 8 ) = -e( jsp1, jsp1 )
1006 *
1007 * Set up right hand side(s)
1008 *
1009  k = 1
1010  ii = mb*nb + 1
1011  DO 160 jj = 0, nb - 1
1012  CALL scopy( mb, c( is, js+jj ), 1, rhs( k ), 1 )
1013  CALL scopy( mb, f( is, js+jj ), 1, rhs( ii ), 1 )
1014  k = k + mb
1015  ii = ii + mb
1016  160 CONTINUE
1017 *
1018 *
1019 * Solve Z**T * x = RHS
1020 *
1021  CALL sgetc2( zdim, z, ldz, ipiv, jpiv, ierr )
1022  IF( ierr.GT.0 )
1023  $ info = ierr
1024 *
1025  CALL sgesc2( zdim, z, ldz, rhs, ipiv, jpiv, scaloc )
1026  IF( scaloc.NE.one ) THEN
1027  DO 170 k = 1, n
1028  CALL sscal( m, scaloc, c( 1, k ), 1 )
1029  CALL sscal( m, scaloc, f( 1, k ), 1 )
1030  170 CONTINUE
1031  scale = scale*scaloc
1032  END IF
1033 *
1034 * Unpack solution vector(s)
1035 *
1036  k = 1
1037  ii = mb*nb + 1
1038  DO 180 jj = 0, nb - 1
1039  CALL scopy( mb, rhs( k ), 1, c( is, js+jj ), 1 )
1040  CALL scopy( mb, rhs( ii ), 1, f( is, js+jj ), 1 )
1041  k = k + mb
1042  ii = ii + mb
1043  180 CONTINUE
1044 *
1045 * Substitute R(I, J) and L(I, J) into remaining
1046 * equation.
1047 *
1048  IF( j.GT.p+2 ) THEN
1049  CALL sgemm( 'N', 'T', mb, js-1, nb, one,
1050  $ c( is, js ), ldc, b( 1, js ), ldb, one,
1051  $ f( is, 1 ), ldf )
1052  CALL sgemm( 'N', 'T', mb, js-1, nb, one,
1053  $ f( is, js ), ldf, e( 1, js ), lde, one,
1054  $ f( is, 1 ), ldf )
1055  END IF
1056  IF( i.LT.p ) THEN
1057  CALL sgemm( 'T', 'N', m-ie, nb, mb, -one,
1058  $ a( is, ie+1 ), lda, c( is, js ), ldc,
1059  $ one, c( ie+1, js ), ldc )
1060  CALL sgemm( 'T', 'N', m-ie, nb, mb, -one,
1061  $ d( is, ie+1 ), ldd, f( is, js ), ldf,
1062  $ one, c( ie+1, js ), ldc )
1063  END IF
1064 *
1065  END IF
1066 *
1067  190 CONTINUE
1068  200 CONTINUE
1069 *
1070  END IF
1071  RETURN
1072 *
1073 * End of STGSY2
1074 *
1075  END