LAPACK 3.12.0 LAPACK: Linear Algebra PACKage
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◆ sgegs()

 subroutine sgegs ( character jobvsl, character jobvsr, integer n, real, dimension( lda, * ) a, integer lda, real, dimension( ldb, * ) b, integer ldb, real, dimension( * ) alphar, real, dimension( * ) alphai, real, dimension( * ) beta, real, dimension( ldvsl, * ) vsl, integer ldvsl, real, dimension( ldvsr, * ) vsr, integer ldvsr, real, dimension( * ) work, integer lwork, integer info )

SGEGS computes the eigenvalues, real Schur form, and, optionally, the left and/or right Schur vectors of a real matrix pair (A,B)

Purpose:
``` This routine is deprecated and has been replaced by routine SGGES.

SGEGS computes the eigenvalues, real Schur form, and, optionally,
left and or/right Schur vectors of a real matrix pair (A,B).
Given two square matrices A and B, the generalized real Schur
factorization has the form

A = Q*S*Z**T,  B = Q*T*Z**T

where Q and Z are orthogonal matrices, T is upper triangular, and S
is an upper quasi-triangular matrix with 1-by-1 and 2-by-2 diagonal
blocks, the 2-by-2 blocks corresponding to complex conjugate pairs
of eigenvalues of (A,B).  The columns of Q are the left Schur vectors
and the columns of Z are the right Schur vectors.

If only the eigenvalues of (A,B) are needed, the driver routine
SGEGV should be used instead.  See SGEGV for a description of the
eigenvalues of the generalized nonsymmetric eigenvalue problem
(GNEP).```
Parameters
 [in] JOBVSL ``` JOBVSL is CHARACTER*1 = 'N': do not compute the left Schur vectors; = 'V': compute the left Schur vectors (returned in VSL).``` [in] JOBVSR ``` JOBVSR is CHARACTER*1 = 'N': do not compute the right Schur vectors; = 'V': compute the right Schur vectors (returned in VSR).``` [in] N ``` N is INTEGER The order of the matrices A, B, VSL, and VSR. N >= 0.``` [in,out] A ``` A is REAL array, dimension (LDA, N) On entry, the matrix A. On exit, the upper quasi-triangular matrix S from the generalized real Schur factorization.``` [in] LDA ``` LDA is INTEGER The leading dimension of A. LDA >= max(1,N).``` [in,out] B ``` B is REAL array, dimension (LDB, N) On entry, the matrix B. On exit, the upper triangular matrix T from the generalized real Schur factorization.``` [in] LDB ``` LDB is INTEGER The leading dimension of B. LDB >= max(1,N).``` [out] ALPHAR ``` ALPHAR is REAL array, dimension (N) The real parts of each scalar alpha defining an eigenvalue of GNEP.``` [out] ALPHAI ``` ALPHAI is REAL array, dimension (N) The imaginary parts of each scalar alpha defining an eigenvalue of GNEP. If ALPHAI(j) is zero, then the j-th eigenvalue is real; if positive, then the j-th and (j+1)-st eigenvalues are a complex conjugate pair, with ALPHAI(j+1) = -ALPHAI(j).``` [out] BETA ``` BETA is REAL array, dimension (N) The scalars beta that define the eigenvalues of GNEP. Together, the quantities alpha = (ALPHAR(j),ALPHAI(j)) and beta = BETA(j) represent the j-th eigenvalue of the matrix pair (A,B), in one of the forms lambda = alpha/beta or mu = beta/alpha. Since either lambda or mu may overflow, they should not, in general, be computed.``` [out] VSL ``` VSL is REAL array, dimension (LDVSL,N) If JOBVSL = 'V', the matrix of left Schur vectors Q. Not referenced if JOBVSL = 'N'.``` [in] LDVSL ``` LDVSL is INTEGER The leading dimension of the matrix VSL. LDVSL >=1, and if JOBVSL = 'V', LDVSL >= N.``` [out] VSR ``` VSR is REAL array, dimension (LDVSR,N) If JOBVSR = 'V', the matrix of right Schur vectors Z. Not referenced if JOBVSR = 'N'.``` [in] LDVSR ``` LDVSR is INTEGER The leading dimension of the matrix VSR. LDVSR >= 1, and if JOBVSR = 'V', LDVSR >= N.``` [out] WORK ``` WORK is REAL array, dimension (MAX(1,LWORK)) On exit, if INFO = 0, WORK(1) returns the optimal LWORK.``` [in] LWORK ``` LWORK is INTEGER The dimension of the array WORK. LWORK >= max(1,4*N). For good performance, LWORK must generally be larger. To compute the optimal value of LWORK, call ILAENV to get blocksizes (for SGEQRF, SORMQR, and SORGQR.) Then compute: NB -- MAX of the blocksizes for SGEQRF, SORMQR, and SORGQR The optimal LWORK is 2*N + N*(NB+1). If LWORK = -1, then a workspace query is assumed; the routine only calculates the optimal size of the WORK array, returns this value as the first entry of the WORK array, and no error message related to LWORK is issued by XERBLA.``` [out] INFO ``` INFO is INTEGER = 0: successful exit < 0: if INFO = -i, the i-th argument had an illegal value. = 1,...,N: The QZ iteration failed. (A,B) are not in Schur form, but ALPHAR(j), ALPHAI(j), and BETA(j) should be correct for j=INFO+1,...,N. > N: errors that usually indicate LAPACK problems: =N+1: error return from SGGBAL =N+2: error return from SGEQRF =N+3: error return from SORMQR =N+4: error return from SORGQR =N+5: error return from SGGHRD =N+6: error return from SHGEQZ (other than failed iteration) =N+7: error return from SGGBAK (computing VSL) =N+8: error return from SGGBAK (computing VSR) =N+9: error return from SLASCL (various places)```

Definition at line 224 of file sgegs.f.

227*
228* -- LAPACK driver routine --
229* -- LAPACK is a software package provided by Univ. of Tennessee, --
230* -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
231*
232* .. Scalar Arguments ..
233 CHARACTER JOBVSL, JOBVSR
234 INTEGER INFO, LDA, LDB, LDVSL, LDVSR, LWORK, N
235* ..
236* .. Array Arguments ..
237 REAL A( LDA, * ), ALPHAI( * ), ALPHAR( * ),
238 \$ B( LDB, * ), BETA( * ), VSL( LDVSL, * ),
239 \$ VSR( LDVSR, * ), WORK( * )
240* ..
241*
242* =====================================================================
243*
244* .. Parameters ..
245 REAL ZERO, ONE
246 parameter( zero = 0.0e0, one = 1.0e0 )
247* ..
248* .. Local Scalars ..
249 LOGICAL ILASCL, ILBSCL, ILVSL, ILVSR, LQUERY
250 INTEGER ICOLS, IHI, IINFO, IJOBVL, IJOBVR, ILEFT,
251 \$ ILO, IRIGHT, IROWS, ITAU, IWORK, LOPT, LWKMIN,
252 \$ LWKOPT, NB, NB1, NB2, NB3
253 REAL ANRM, ANRMTO, BIGNUM, BNRM, BNRMTO, EPS,
254 \$ SAFMIN, SMLNUM
255* ..
256* .. External Subroutines ..
257 EXTERNAL sgeqrf, sggbak, sggbal, sgghrd, shgeqz, slacpy,
259* ..
260* .. External Functions ..
261 LOGICAL LSAME
262 INTEGER ILAENV
263 REAL SLAMCH, SLANGE
264 EXTERNAL ilaenv, lsame, slamch, slange
265* ..
266* .. Intrinsic Functions ..
267 INTRINSIC int, max
268* ..
269* .. Executable Statements ..
270*
271* Decode the input arguments
272*
273 IF( lsame( jobvsl, 'N' ) ) THEN
274 ijobvl = 1
275 ilvsl = .false.
276 ELSE IF( lsame( jobvsl, 'V' ) ) THEN
277 ijobvl = 2
278 ilvsl = .true.
279 ELSE
280 ijobvl = -1
281 ilvsl = .false.
282 END IF
283*
284 IF( lsame( jobvsr, 'N' ) ) THEN
285 ijobvr = 1
286 ilvsr = .false.
287 ELSE IF( lsame( jobvsr, 'V' ) ) THEN
288 ijobvr = 2
289 ilvsr = .true.
290 ELSE
291 ijobvr = -1
292 ilvsr = .false.
293 END IF
294*
295* Test the input arguments
296*
297 lwkmin = max( 4*n, 1 )
298 lwkopt = lwkmin
299 work( 1 ) = lwkopt
300 lquery = ( lwork.EQ.-1 )
301 info = 0
302 IF( ijobvl.LE.0 ) THEN
303 info = -1
304 ELSE IF( ijobvr.LE.0 ) THEN
305 info = -2
306 ELSE IF( n.LT.0 ) THEN
307 info = -3
308 ELSE IF( lda.LT.max( 1, n ) ) THEN
309 info = -5
310 ELSE IF( ldb.LT.max( 1, n ) ) THEN
311 info = -7
312 ELSE IF( ldvsl.LT.1 .OR. ( ilvsl .AND. ldvsl.LT.n ) ) THEN
313 info = -12
314 ELSE IF( ldvsr.LT.1 .OR. ( ilvsr .AND. ldvsr.LT.n ) ) THEN
315 info = -14
316 ELSE IF( lwork.LT.lwkmin .AND. .NOT.lquery ) THEN
317 info = -16
318 END IF
319*
320 IF( info.EQ.0 ) THEN
321 nb1 = ilaenv( 1, 'SGEQRF', ' ', n, n, -1, -1 )
322 nb2 = ilaenv( 1, 'SORMQR', ' ', n, n, n, -1 )
323 nb3 = ilaenv( 1, 'SORGQR', ' ', n, n, n, -1 )
324 nb = max( nb1, nb2, nb3 )
325 lopt = 2*n+n*(nb+1)
326 work( 1 ) = lopt
327 END IF
328*
329 IF( info.NE.0 ) THEN
330 CALL xerbla( 'SGEGS ', -info )
331 RETURN
332 ELSE IF( lquery ) THEN
333 RETURN
334 END IF
335*
336* Quick return if possible
337*
338 IF( n.EQ.0 )
339 \$ RETURN
340*
341* Get machine constants
342*
343 eps = slamch( 'E' )*slamch( 'B' )
344 safmin = slamch( 'S' )
345 smlnum = n*safmin / eps
346 bignum = one / smlnum
347*
348* Scale A if max element outside range [SMLNUM,BIGNUM]
349*
350 anrm = slange( 'M', n, n, a, lda, work )
351 ilascl = .false.
352 IF( anrm.GT.zero .AND. anrm.LT.smlnum ) THEN
353 anrmto = smlnum
354 ilascl = .true.
355 ELSE IF( anrm.GT.bignum ) THEN
356 anrmto = bignum
357 ilascl = .true.
358 END IF
359*
360 IF( ilascl ) THEN
361 CALL slascl( 'G', -1, -1, anrm, anrmto, n, n, a, lda, iinfo )
362 IF( iinfo.NE.0 ) THEN
363 info = n + 9
364 RETURN
365 END IF
366 END IF
367*
368* Scale B if max element outside range [SMLNUM,BIGNUM]
369*
370 bnrm = slange( 'M', n, n, b, ldb, work )
371 ilbscl = .false.
372 IF( bnrm.GT.zero .AND. bnrm.LT.smlnum ) THEN
373 bnrmto = smlnum
374 ilbscl = .true.
375 ELSE IF( bnrm.GT.bignum ) THEN
376 bnrmto = bignum
377 ilbscl = .true.
378 END IF
379*
380 IF( ilbscl ) THEN
381 CALL slascl( 'G', -1, -1, bnrm, bnrmto, n, n, b, ldb, iinfo )
382 IF( iinfo.NE.0 ) THEN
383 info = n + 9
384 RETURN
385 END IF
386 END IF
387*
388* Permute the matrix to make it more nearly triangular
389* Workspace layout: (2*N words -- "work..." not actually used)
390* left_permutation, right_permutation, work...
391*
392 ileft = 1
393 iright = n + 1
394 iwork = iright + n
395 CALL sggbal( 'P', n, a, lda, b, ldb, ilo, ihi, work( ileft ),
396 \$ work( iright ), work( iwork ), iinfo )
397 IF( iinfo.NE.0 ) THEN
398 info = n + 1
399 GO TO 10
400 END IF
401*
402* Reduce B to triangular form, and initialize VSL and/or VSR
403* Workspace layout: ("work..." must have at least N words)
404* left_permutation, right_permutation, tau, work...
405*
406 irows = ihi + 1 - ilo
407 icols = n + 1 - ilo
408 itau = iwork
409 iwork = itau + irows
410 CALL sgeqrf( irows, icols, b( ilo, ilo ), ldb, work( itau ),
411 \$ work( iwork ), lwork+1-iwork, iinfo )
412 IF( iinfo.GE.0 )
413 \$ lwkopt = max( lwkopt, int( work( iwork ) )+iwork-1 )
414 IF( iinfo.NE.0 ) THEN
415 info = n + 2
416 GO TO 10
417 END IF
418*
419 CALL sormqr( 'L', 'T', irows, icols, irows, b( ilo, ilo ), ldb,
420 \$ work( itau ), a( ilo, ilo ), lda, work( iwork ),
421 \$ lwork+1-iwork, iinfo )
422 IF( iinfo.GE.0 )
423 \$ lwkopt = max( lwkopt, int( work( iwork ) )+iwork-1 )
424 IF( iinfo.NE.0 ) THEN
425 info = n + 3
426 GO TO 10
427 END IF
428*
429 IF( ilvsl ) THEN
430 CALL slaset( 'Full', n, n, zero, one, vsl, ldvsl )
431 CALL slacpy( 'L', irows-1, irows-1, b( ilo+1, ilo ), ldb,
432 \$ vsl( ilo+1, ilo ), ldvsl )
433 CALL sorgqr( irows, irows, irows, vsl( ilo, ilo ), ldvsl,
434 \$ work( itau ), work( iwork ), lwork+1-iwork,
435 \$ iinfo )
436 IF( iinfo.GE.0 )
437 \$ lwkopt = max( lwkopt, int( work( iwork ) )+iwork-1 )
438 IF( iinfo.NE.0 ) THEN
439 info = n + 4
440 GO TO 10
441 END IF
442 END IF
443*
444 IF( ilvsr )
445 \$ CALL slaset( 'Full', n, n, zero, one, vsr, ldvsr )
446*
447* Reduce to generalized Hessenberg form
448*
449 CALL sgghrd( jobvsl, jobvsr, n, ilo, ihi, a, lda, b, ldb, vsl,
450 \$ ldvsl, vsr, ldvsr, iinfo )
451 IF( iinfo.NE.0 ) THEN
452 info = n + 5
453 GO TO 10
454 END IF
455*
456* Perform QZ algorithm, computing Schur vectors if desired
457* Workspace layout: ("work..." must have at least 1 word)
458* left_permutation, right_permutation, work...
459*
460 iwork = itau
461 CALL shgeqz( 'S', jobvsl, jobvsr, n, ilo, ihi, a, lda, b, ldb,
462 \$ alphar, alphai, beta, vsl, ldvsl, vsr, ldvsr,
463 \$ work( iwork ), lwork+1-iwork, iinfo )
464 IF( iinfo.GE.0 )
465 \$ lwkopt = max( lwkopt, int( work( iwork ) )+iwork-1 )
466 IF( iinfo.NE.0 ) THEN
467 IF( iinfo.GT.0 .AND. iinfo.LE.n ) THEN
468 info = iinfo
469 ELSE IF( iinfo.GT.n .AND. iinfo.LE.2*n ) THEN
470 info = iinfo - n
471 ELSE
472 info = n + 6
473 END IF
474 GO TO 10
475 END IF
476*
477* Apply permutation to VSL and VSR
478*
479 IF( ilvsl ) THEN
480 CALL sggbak( 'P', 'L', n, ilo, ihi, work( ileft ),
481 \$ work( iright ), n, vsl, ldvsl, iinfo )
482 IF( iinfo.NE.0 ) THEN
483 info = n + 7
484 GO TO 10
485 END IF
486 END IF
487 IF( ilvsr ) THEN
488 CALL sggbak( 'P', 'R', n, ilo, ihi, work( ileft ),
489 \$ work( iright ), n, vsr, ldvsr, iinfo )
490 IF( iinfo.NE.0 ) THEN
491 info = n + 8
492 GO TO 10
493 END IF
494 END IF
495*
496* Undo scaling
497*
498 IF( ilascl ) THEN
499 CALL slascl( 'H', -1, -1, anrmto, anrm, n, n, a, lda, iinfo )
500 IF( iinfo.NE.0 ) THEN
501 info = n + 9
502 RETURN
503 END IF
504 CALL slascl( 'G', -1, -1, anrmto, anrm, n, 1, alphar, n,
505 \$ iinfo )
506 IF( iinfo.NE.0 ) THEN
507 info = n + 9
508 RETURN
509 END IF
510 CALL slascl( 'G', -1, -1, anrmto, anrm, n, 1, alphai, n,
511 \$ iinfo )
512 IF( iinfo.NE.0 ) THEN
513 info = n + 9
514 RETURN
515 END IF
516 END IF
517*
518 IF( ilbscl ) THEN
519 CALL slascl( 'U', -1, -1, bnrmto, bnrm, n, n, b, ldb, iinfo )
520 IF( iinfo.NE.0 ) THEN
521 info = n + 9
522 RETURN
523 END IF
524 CALL slascl( 'G', -1, -1, bnrmto, bnrm, n, 1, beta, n, iinfo )
525 IF( iinfo.NE.0 ) THEN
526 info = n + 9
527 RETURN
528 END IF
529 END IF
530*
531 10 CONTINUE
532 work( 1 ) = lwkopt
533*
534 RETURN
535*
536* End of SGEGS
537*
subroutine xerbla(srname, info)
Definition cblat2.f:3285
subroutine sgeqrf(m, n, a, lda, tau, work, lwork, info)
SGEQRF
Definition sgeqrf.f:146
subroutine sggbak(job, side, n, ilo, ihi, lscale, rscale, m, v, ldv, info)
SGGBAK
Definition sggbak.f:147
subroutine sggbal(job, n, a, lda, b, ldb, ilo, ihi, lscale, rscale, work, info)
SGGBAL
Definition sggbal.f:177
subroutine sgghrd(compq, compz, n, ilo, ihi, a, lda, b, ldb, q, ldq, z, ldz, info)
SGGHRD
Definition sgghrd.f:207
subroutine shgeqz(job, compq, compz, n, ilo, ihi, h, ldh, t, ldt, alphar, alphai, beta, q, ldq, z, ldz, work, lwork, info)
SHGEQZ
Definition shgeqz.f:304
integer function ilaenv(ispec, name, opts, n1, n2, n3, n4)
ILAENV
Definition ilaenv.f:162
subroutine slacpy(uplo, m, n, a, lda, b, ldb)
SLACPY copies all or part of one two-dimensional array to another.
Definition slacpy.f:103
real function slamch(cmach)
SLAMCH
Definition slamch.f:68
real function slange(norm, m, n, a, lda, work)
SLANGE returns the value of the 1-norm, Frobenius norm, infinity-norm, or the largest absolute value ...
Definition slange.f:114
subroutine slascl(type, kl, ku, cfrom, cto, m, n, a, lda, info)
SLASCL multiplies a general rectangular matrix by a real scalar defined as cto/cfrom.
Definition slascl.f:143
subroutine slaset(uplo, m, n, alpha, beta, a, lda)
SLASET initializes the off-diagonal elements and the diagonal elements of a matrix to given values.
Definition slaset.f:110
logical function lsame(ca, cb)
LSAME
Definition lsame.f:48
subroutine sorgqr(m, n, k, a, lda, tau, work, lwork, info)
SORGQR
Definition sorgqr.f:128
subroutine sormqr(side, trans, m, n, k, a, lda, tau, c, ldc, work, lwork, info)
SORMQR
Definition sormqr.f:168
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