LAPACK  3.10.0
LAPACK: Linear Algebra PACKage

◆ sggbak()

subroutine sggbak ( character  JOB,
character  SIDE,
integer  N,
integer  ILO,
integer  IHI,
real, dimension( * )  LSCALE,
real, dimension( * )  RSCALE,
integer  M,
real, dimension( ldv, * )  V,
integer  LDV,
integer  INFO 
)

SGGBAK

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Purpose:
 SGGBAK forms the right or left eigenvectors of a real generalized
 eigenvalue problem A*x = lambda*B*x, by backward transformation on
 the computed eigenvectors of the balanced pair of matrices output by
 SGGBAL.
Parameters
[in]JOB
          JOB is CHARACTER*1
          Specifies the type of backward transformation required:
          = 'N':  do nothing, return immediately;
          = 'P':  do backward transformation for permutation only;
          = 'S':  do backward transformation for scaling only;
          = 'B':  do backward transformations for both permutation and
                  scaling.
          JOB must be the same as the argument JOB supplied to SGGBAL.
[in]SIDE
          SIDE is CHARACTER*1
          = 'R':  V contains right eigenvectors;
          = 'L':  V contains left eigenvectors.
[in]N
          N is INTEGER
          The number of rows of the matrix V.  N >= 0.
[in]ILO
          ILO is INTEGER
[in]IHI
          IHI is INTEGER
          The integers ILO and IHI determined by SGGBAL.
          1 <= ILO <= IHI <= N, if N > 0; ILO=1 and IHI=0, if N=0.
[in]LSCALE
          LSCALE is REAL array, dimension (N)
          Details of the permutations and/or scaling factors applied
          to the left side of A and B, as returned by SGGBAL.
[in]RSCALE
          RSCALE is REAL array, dimension (N)
          Details of the permutations and/or scaling factors applied
          to the right side of A and B, as returned by SGGBAL.
[in]M
          M is INTEGER
          The number of columns of the matrix V.  M >= 0.
[in,out]V
          V is REAL array, dimension (LDV,M)
          On entry, the matrix of right or left eigenvectors to be
          transformed, as returned by STGEVC.
          On exit, V is overwritten by the transformed eigenvectors.
[in]LDV
          LDV is INTEGER
          The leading dimension of the matrix V. LDV >= max(1,N).
[out]INFO
          INFO is INTEGER
          = 0:  successful exit.
          < 0:  if INFO = -i, the i-th argument had an illegal value.
Author
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.
Further Details:
  See R.C. Ward, Balancing the generalized eigenvalue problem,
                 SIAM J. Sci. Stat. Comp. 2 (1981), 141-152.

Definition at line 145 of file sggbak.f.

147 *
148 * -- LAPACK computational routine --
149 * -- LAPACK is a software package provided by Univ. of Tennessee, --
150 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
151 *
152 * .. Scalar Arguments ..
153  CHARACTER JOB, SIDE
154  INTEGER IHI, ILO, INFO, LDV, M, N
155 * ..
156 * .. Array Arguments ..
157  REAL LSCALE( * ), RSCALE( * ), V( LDV, * )
158 * ..
159 *
160 * =====================================================================
161 *
162 * .. Local Scalars ..
163  LOGICAL LEFTV, RIGHTV
164  INTEGER I, K
165 * ..
166 * .. External Functions ..
167  LOGICAL LSAME
168  EXTERNAL lsame
169 * ..
170 * .. External Subroutines ..
171  EXTERNAL sscal, sswap, xerbla
172 * ..
173 * .. Intrinsic Functions ..
174  INTRINSIC max
175 * ..
176 * .. Executable Statements ..
177 *
178 * Test the input parameters
179 *
180  rightv = lsame( side, 'R' )
181  leftv = lsame( side, 'L' )
182 *
183  info = 0
184  IF( .NOT.lsame( job, 'N' ) .AND. .NOT.lsame( job, 'P' ) .AND.
185  $ .NOT.lsame( job, 'S' ) .AND. .NOT.lsame( job, 'B' ) ) THEN
186  info = -1
187  ELSE IF( .NOT.rightv .AND. .NOT.leftv ) THEN
188  info = -2
189  ELSE IF( n.LT.0 ) THEN
190  info = -3
191  ELSE IF( ilo.LT.1 ) THEN
192  info = -4
193  ELSE IF( n.EQ.0 .AND. ihi.EQ.0 .AND. ilo.NE.1 ) THEN
194  info = -4
195  ELSE IF( n.GT.0 .AND. ( ihi.LT.ilo .OR. ihi.GT.max( 1, n ) ) )
196  $ THEN
197  info = -5
198  ELSE IF( n.EQ.0 .AND. ilo.EQ.1 .AND. ihi.NE.0 ) THEN
199  info = -5
200  ELSE IF( m.LT.0 ) THEN
201  info = -8
202  ELSE IF( ldv.LT.max( 1, n ) ) THEN
203  info = -10
204  END IF
205  IF( info.NE.0 ) THEN
206  CALL xerbla( 'SGGBAK', -info )
207  RETURN
208  END IF
209 *
210 * Quick return if possible
211 *
212  IF( n.EQ.0 )
213  $ RETURN
214  IF( m.EQ.0 )
215  $ RETURN
216  IF( lsame( job, 'N' ) )
217  $ RETURN
218 *
219  IF( ilo.EQ.ihi )
220  $ GO TO 30
221 *
222 * Backward balance
223 *
224  IF( lsame( job, 'S' ) .OR. lsame( job, 'B' ) ) THEN
225 *
226 * Backward transformation on right eigenvectors
227 *
228  IF( rightv ) THEN
229  DO 10 i = ilo, ihi
230  CALL sscal( m, rscale( i ), v( i, 1 ), ldv )
231  10 CONTINUE
232  END IF
233 *
234 * Backward transformation on left eigenvectors
235 *
236  IF( leftv ) THEN
237  DO 20 i = ilo, ihi
238  CALL sscal( m, lscale( i ), v( i, 1 ), ldv )
239  20 CONTINUE
240  END IF
241  END IF
242 *
243 * Backward permutation
244 *
245  30 CONTINUE
246  IF( lsame( job, 'P' ) .OR. lsame( job, 'B' ) ) THEN
247 *
248 * Backward permutation on right eigenvectors
249 *
250  IF( rightv ) THEN
251  IF( ilo.EQ.1 )
252  $ GO TO 50
253 *
254  DO 40 i = ilo - 1, 1, -1
255  k = rscale( i )
256  IF( k.EQ.i )
257  $ GO TO 40
258  CALL sswap( m, v( i, 1 ), ldv, v( k, 1 ), ldv )
259  40 CONTINUE
260 *
261  50 CONTINUE
262  IF( ihi.EQ.n )
263  $ GO TO 70
264  DO 60 i = ihi + 1, n
265  k = rscale( i )
266  IF( k.EQ.i )
267  $ GO TO 60
268  CALL sswap( m, v( i, 1 ), ldv, v( k, 1 ), ldv )
269  60 CONTINUE
270  END IF
271 *
272 * Backward permutation on left eigenvectors
273 *
274  70 CONTINUE
275  IF( leftv ) THEN
276  IF( ilo.EQ.1 )
277  $ GO TO 90
278  DO 80 i = ilo - 1, 1, -1
279  k = lscale( i )
280  IF( k.EQ.i )
281  $ GO TO 80
282  CALL sswap( m, v( i, 1 ), ldv, v( k, 1 ), ldv )
283  80 CONTINUE
284 *
285  90 CONTINUE
286  IF( ihi.EQ.n )
287  $ GO TO 110
288  DO 100 i = ihi + 1, n
289  k = lscale( i )
290  IF( k.EQ.i )
291  $ GO TO 100
292  CALL sswap( m, v( i, 1 ), ldv, v( k, 1 ), ldv )
293  100 CONTINUE
294  END IF
295  END IF
296 *
297  110 CONTINUE
298 *
299  RETURN
300 *
301 * End of SGGBAK
302 *
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:60
logical function lsame(CA, CB)
LSAME
Definition: lsame.f:53
subroutine sswap(N, SX, INCX, SY, INCY)
SSWAP
Definition: sswap.f:82
subroutine sscal(N, SA, SX, INCX)
SSCAL
Definition: sscal.f:79
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