sgees()

subroutine sgees	(	character	jobvs,
		character	sort,
		external	select,
		integer	n,
		real, dimension( lda, * )	a,
		integer	lda,
		integer	sdim,
		real, dimension( * )	wr,
		real, dimension( * )	wi,
		real, dimension( ldvs, * )	vs,
		integer	ldvs,
		real, dimension( * )	work,
		integer	lwork,
		logical, dimension( * )	bwork,
		integer	info )

SGEES computes the eigenvalues, the Schur form, and, optionally, the matrix of Schur vectors for GE matrices

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Purpose:

!>
!> SGEES computes for an N-by-N real nonsymmetric matrix A, the
!> eigenvalues, the real Schur form T, and, optionally, the matrix of
!> Schur vectors Z.  This gives the Schur factorization A = Z*T*(Z**T).
!>
!> Optionally, it also orders the eigenvalues on the diagonal of the
!> real Schur form so that selected eigenvalues are at the top left.
!> The leading columns of Z then form an orthonormal basis for the
!> invariant subspace corresponding to the selected eigenvalues.
!>
!> A matrix is in real Schur form if it is upper quasi-triangular with
!> 1-by-1 and 2-by-2 blocks. 2-by-2 blocks will be standardized in the
!> form
!>         [  a  b  ]
!>         [  c  a  ]
!>
!> where b*c < 0. The eigenvalues of such a block are a +- sqrt(bc).
!> 

Parameters

[in]	JOBVS	!> JOBVS is CHARACTER*1 !> = 'N': Schur vectors are not computed; !> = 'V': Schur vectors are computed. !>
[in]	SORT	!> SORT is CHARACTER*1 !> Specifies whether or not to order the eigenvalues on the !> diagonal of the Schur form. !> = 'N': Eigenvalues are not ordered; !> = 'S': Eigenvalues are ordered (see SELECT). !>
[in]	SELECT	!> SELECT is a LOGICAL FUNCTION of two REAL arguments !> SELECT must be declared EXTERNAL in the calling subroutine. !> If SORT = 'S', SELECT is used to select eigenvalues to sort !> to the top left of the Schur form. !> If SORT = 'N', SELECT is not referenced. !> An eigenvalue WR(j)+sqrt(-1)*WI(j) is selected if !> SELECT(WR(j),WI(j)) is true; i.e., if either one of a complex !> conjugate pair of eigenvalues is selected, then both complex !> eigenvalues are selected. !> Note that a selected complex eigenvalue may no longer !> satisfy SELECT(WR(j),WI(j)) = .TRUE. after ordering, since !> ordering may change the value of complex eigenvalues !> (especially if the eigenvalue is ill-conditioned); in this !> case INFO is set to N+2 (see INFO below). !>
[in]	N	!> N is INTEGER !> The order of the matrix A. N >= 0. !>
[in,out]	A	!> A is REAL array, dimension (LDA,N) !> On entry, the N-by-N matrix A. !> On exit, A has been overwritten by its real Schur form T. !>
[in]	LDA	!> LDA is INTEGER !> The leading dimension of the array A. LDA >= max(1,N). !>
[out]	SDIM	!> SDIM is INTEGER !> If SORT = 'N', SDIM = 0. !> If SORT = 'S', SDIM = number of eigenvalues (after sorting) !> for which SELECT is true. (Complex conjugate !> pairs for which SELECT is true for either !> eigenvalue count as 2.) !>
[out]	WR	!> WR is REAL array, dimension (N) !>
[out]	WI	!> WI is REAL array, dimension (N) !> WR and WI contain the real and imaginary parts, !> respectively, of the computed eigenvalues in the same order !> that they appear on the diagonal of the output Schur form T. !> Complex conjugate pairs of eigenvalues will appear !> consecutively with the eigenvalue having the positive !> imaginary part first. !>
[out]	VS	!> VS is REAL array, dimension (LDVS,N) !> If JOBVS = 'V', VS contains the orthogonal matrix Z of Schur !> vectors. !> If JOBVS = 'N', VS is not referenced. !>
[in]	LDVS	!> LDVS is INTEGER !> The leading dimension of the array VS. LDVS >= 1; if !> JOBVS = 'V', LDVS >= N. !>
[out]	WORK	!> WORK is REAL array, dimension (MAX(1,LWORK)) !> On exit, if INFO = 0, WORK(1) contains the optimal LWORK. !>
[in]	LWORK	!> LWORK is INTEGER !> The dimension of the array WORK. LWORK >= max(1,3*N). !> For good performance, LWORK must generally be larger. !> !> 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]	BWORK	!> BWORK is LOGICAL array, dimension (N) !> Not referenced if SORT = 'N'. !>
[out]	INFO	!> INFO is INTEGER !> = 0: successful exit !> < 0: if INFO = -i, the i-th argument had an illegal value. !> > 0: if INFO = i, and i is !> <= N: the QR algorithm failed to compute all the !> eigenvalues; elements 1:ILO-1 and i+1:N of WR and WI !> contain those eigenvalues which have converged; if !> JOBVS = 'V', VS contains the matrix which reduces A !> to its partially converged Schur form. !> = N+1: the eigenvalues could not be reordered because some !> eigenvalues were too close to separate (the problem !> is very ill-conditioned); !> = N+2: after reordering, roundoff changed values of some !> complex eigenvalues so that leading eigenvalues in !> the Schur form no longer satisfy SELECT=.TRUE. This !> could also be caused by underflow due to scaling. !>

Author

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

Definition at line 212 of file sgees.f.

*
*  -- LAPACK driver routine --
*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*
*     .. Scalar Arguments ..
      CHARACTER          JOBVS, SORT
      INTEGER            INFO, LDA, LDVS, LWORK, N, SDIM
*     ..
*     .. Array Arguments ..
      LOGICAL            BWORK( * )
      REAL               A( LDA, * ), VS( LDVS, * ), WI( * ), WORK( * ),
     $                   WR( * )
*     ..
*     .. Function Arguments ..
      LOGICAL            SELECT
      EXTERNAL           SELECT
*     ..
*
*  =====================================================================
*
*     .. Parameters ..
      REAL               ZERO, ONE
      parameter( zero = 0.0e0, one = 1.0e0 )
*     ..
*     .. Local Scalars ..
      LOGICAL            CURSL, LASTSL, LQUERY, LST2SL, SCALEA, WANTST,
     $                   WANTVS
      INTEGER            HSWORK, I, I1, I2, IBAL, ICOND, IERR, IEVAL,
     $                   IHI, ILO, INXT, IP, ITAU, IWRK, MAXWRK, MINWRK
      REAL               ANRM, BIGNUM, CSCALE, EPS, S, SEP, SMLNUM
*     ..
*     .. Local Arrays ..
      INTEGER            IDUM( 1 )
      REAL               DUM( 1 )
*     ..
*     .. External Subroutines ..
      EXTERNAL           scopy, sgebak, sgebal, sgehrd, shseqr,
     $                   slacpy,
     $                   slascl, sorghr, sswap, strsen, xerbla
*     ..
*     .. External Functions ..
      LOGICAL            LSAME
      INTEGER            ILAENV
      REAL               SLAMCH, SLANGE, SROUNDUP_LWORK
      EXTERNAL           lsame, ilaenv, slamch, slange,
     $                   sroundup_lwork
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          max, sqrt
*     ..
*     .. Executable Statements ..
*
*     Test the input arguments
*
      info = 0
      lquery = ( lwork.EQ.-1 )
      wantvs = lsame( jobvs, 'V' )
      wantst = lsame( sort, 'S' )
      IF( ( .NOT.wantvs ) .AND. ( .NOT.lsame( jobvs, 'N' ) ) ) THEN
         info = -1
      ELSE IF( ( .NOT.wantst ) .AND.
     $         ( .NOT.lsame( sort, 'N' ) ) ) THEN
         info = -2
      ELSE IF( n.LT.0 ) THEN
         info = -4
      ELSE IF( lda.LT.max( 1, n ) ) THEN
         info = -6
      ELSE IF( ldvs.LT.1 .OR. ( wantvs .AND. ldvs.LT.n ) ) THEN
         info = -11
      END IF
*
*     Compute workspace
*      (Note: Comments in the code beginning "Workspace:" describe the
*       minimal amount of workspace needed at that point in the code,
*       as well as the preferred amount for good performance.
*       NB refers to the optimal block size for the immediately
*       following subroutine, as returned by ILAENV.
*       HSWORK refers to the workspace preferred by SHSEQR, as
*       calculated below. HSWORK is computed assuming ILO=1 and IHI=N,
*       the worst case.)
*
      IF( info.EQ.0 ) THEN
         IF( n.EQ.0 ) THEN
            minwrk = 1
            maxwrk = 1
         ELSE
            maxwrk = 2*n + n*ilaenv( 1, 'SGEHRD', ' ', n, 1, n, 0 )
            minwrk = 3*n
*
            CALL shseqr( 'S', jobvs, n, 1, n, a, lda, wr, wi, vs,
     $                   ldvs,
     $             work, -1, ieval )
            hswork = int( work( 1 ) )
*
            IF( .NOT.wantvs ) THEN
               maxwrk = max( maxwrk, n + hswork )
            ELSE
               maxwrk = max( maxwrk, 2*n + ( n - 1 )*ilaenv( 1,
     $                       'SORGHR', ' ', n, 1, n, -1 ) )
               maxwrk = max( maxwrk, n + hswork )
            END IF
         END IF
         work( 1 ) = sroundup_lwork(maxwrk)
*
         IF( lwork.LT.minwrk .AND. .NOT.lquery ) THEN
            info = -13
         END IF
      END IF
*
      IF( info.NE.0 ) THEN
         CALL xerbla( 'SGEES ', -info )
         RETURN
      ELSE IF( lquery ) THEN
         RETURN
      END IF
*
*     Quick return if possible
*
      IF( n.EQ.0 ) THEN
         sdim = 0
         RETURN
      END IF
*
*     Get machine constants
*
      eps = slamch( 'P' )
      smlnum = slamch( 'S' )
      bignum = one / smlnum
      smlnum = sqrt( smlnum ) / eps
      bignum = one / smlnum
*
*     Scale A if max element outside range [SMLNUM,BIGNUM]
*
      anrm = slange( 'M', n, n, a, lda, dum )
      scalea = .false.
      IF( anrm.GT.zero .AND. anrm.LT.smlnum ) THEN
         scalea = .true.
         cscale = smlnum
      ELSE IF( anrm.GT.bignum ) THEN
         scalea = .true.
         cscale = bignum
      END IF
      IF( scalea )
     $   CALL slascl( 'G', 0, 0, anrm, cscale, n, n, a, lda, ierr )
*
*     Permute the matrix to make it more nearly triangular
*     (Workspace: need N)
*
      ibal = 1
      CALL sgebal( 'P', n, a, lda, ilo, ihi, work( ibal ), ierr )
*
*     Reduce to upper Hessenberg form
*     (Workspace: need 3*N, prefer 2*N+N*NB)
*
      itau = n + ibal
      iwrk = n + itau
      CALL sgehrd( n, ilo, ihi, a, lda, work( itau ), work( iwrk ),
     $             lwork-iwrk+1, ierr )
*
      IF( wantvs ) THEN
*
*        Copy Householder vectors to VS
*
         CALL slacpy( 'L', n, n, a, lda, vs, ldvs )
*
*        Generate orthogonal matrix in VS
*        (Workspace: need 3*N-1, prefer 2*N+(N-1)*NB)
*
         CALL sorghr( n, ilo, ihi, vs, ldvs, work( itau ),
     $                work( iwrk ),
     $                lwork-iwrk+1, ierr )
      END IF
*
      sdim = 0
*
*     Perform QR iteration, accumulating Schur vectors in VS if desired
*     (Workspace: need N+1, prefer N+HSWORK (see comments) )
*
      iwrk = itau
      CALL shseqr( 'S', jobvs, n, ilo, ihi, a, lda, wr, wi, vs, ldvs,
     $             work( iwrk ), lwork-iwrk+1, ieval )
      IF( ieval.GT.0 )
     $   info = ieval
*
*     Sort eigenvalues if desired
*
      IF( wantst .AND. info.EQ.0 ) THEN
         IF( scalea ) THEN
            CALL slascl( 'G', 0, 0, cscale, anrm, n, 1, wr, n, ierr )
            CALL slascl( 'G', 0, 0, cscale, anrm, n, 1, wi, n, ierr )
         END IF
         DO 10 i = 1, n
            bwork( i ) = SELECT( wr( i ), wi( i ) )
   10    CONTINUE
*
*        Reorder eigenvalues and transform Schur vectors
*        (Workspace: none needed)
*
         CALL strsen( 'N', jobvs, bwork, n, a, lda, vs, ldvs, wr, wi,
     $                sdim, s, sep, work( iwrk ), lwork-iwrk+1, idum, 1,
     $                icond )
         IF( icond.GT.0 )
     $      info = n + icond
      END IF
*
      IF( wantvs ) THEN
*
*        Undo balancing
*        (Workspace: need N)
*
         CALL sgebak( 'P', 'R', n, ilo, ihi, work( ibal ), n, vs,
     $                ldvs,
     $                ierr )
      END IF
*
      IF( scalea ) THEN
*
*        Undo scaling for the Schur form of A
*
         CALL slascl( 'H', 0, 0, cscale, anrm, n, n, a, lda, ierr )
         CALL scopy( n, a, lda+1, wr, 1 )
         IF( cscale.EQ.smlnum ) THEN
*
*           If scaling back towards underflow, adjust WI if an
*           offdiagonal element of a 2-by-2 block in the Schur form
*           underflows.
*
            IF( ieval.GT.0 ) THEN
               i1 = ieval + 1
               i2 = ihi - 1
               CALL slascl( 'G', 0, 0, cscale, anrm, ilo-1, 1, wi,
     $                      max( ilo-1, 1 ), ierr )
            ELSE IF( wantst ) THEN
               i1 = 1
               i2 = n - 1
            ELSE
               i1 = ilo
               i2 = ihi - 1
            END IF
            inxt = i1 - 1
            DO 20 i = i1, i2
               IF( i.LT.inxt )
     $            GO TO 20
               IF( wi( i ).EQ.zero ) THEN
                  inxt = i + 1
               ELSE
                  IF( a( i+1, i ).EQ.zero ) THEN
                     wi( i ) = zero
                     wi( i+1 ) = zero
                  ELSE IF( a( i+1, i ).NE.zero .AND. a( i, i+1 ).EQ.
     $                     zero ) THEN
                     wi( i ) = zero
                     wi( i+1 ) = zero
                     IF( i.GT.1 )
     $                  CALL sswap( i-1, a( 1, i ), 1, a( 1, i+1 ),
     $                              1 )
                     IF( n.GT.i+1 )
     $                  CALL sswap( n-i-1, a( i, i+2 ), lda,
     $                              a( i+1, i+2 ), lda )
                     IF( wantvs ) THEN
                        CALL sswap( n, vs( 1, i ), 1, vs( 1, i+1 ),
     $                              1 )
                     END IF
                     a( i, i+1 ) = a( i+1, i )
                     a( i+1, i ) = zero
                  END IF
                  inxt = i + 2
               END IF
   20       CONTINUE
         END IF
*
*        Undo scaling for the imaginary part of the eigenvalues
*
         CALL slascl( 'G', 0, 0, cscale, anrm, n-ieval, 1,
     $                wi( ieval+1 ), max( n-ieval, 1 ), ierr )
      END IF
*
      IF( wantst .AND. info.EQ.0 ) THEN
*
*        Check if reordering successful
*
         lastsl = .true.
         lst2sl = .true.
         sdim = 0
         ip = 0
         DO 30 i = 1, n
            cursl = SELECT( wr( i ), wi( i ) )
            IF( wi( i ).EQ.zero ) THEN
               IF( cursl )
     $            sdim = sdim + 1
               ip = 0
               IF( cursl .AND. .NOT.lastsl )
     $            info = n + 2
            ELSE
               IF( ip.EQ.1 ) THEN
*
*                 Last eigenvalue of conjugate pair
*
                  cursl = cursl .OR. lastsl
                  lastsl = cursl
                  IF( cursl )
     $               sdim = sdim + 2
                  ip = -1
                  IF( cursl .AND. .NOT.lst2sl )
     $               info = n + 2
               ELSE
*
*                 First eigenvalue of conjugate pair
*
                  ip = 1
               END IF
            END IF
            lst2sl = lastsl
            lastsl = cursl
   30    CONTINUE
      END IF
*
      work( 1 ) = sroundup_lwork(maxwrk)
      RETURN
*
*     End of SGEES
*

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