SGGES computes for a pair of N-by-N real nonsymmetric matrices (A,B),


 the generalized eigenvalues, the generalized real Schur form (S,T),


 optionally, the left and/or right matrices of Schur vectors (VSL and


 VSR). This gives the generalized Schur factorization


          (A,B) = ( (VSL)*S*(VSR)**T, (VSL)*T*(VSR)**T )


 Optionally, it also orders the eigenvalues so that a selected cluster


 of eigenvalues appears in the leading diagonal blocks of the upper


 quasi-triangular matrix S and the upper triangular matrix T.The


 leading columns of VSL and VSR then form an orthonormal basis for the


 corresponding left and right eigenspaces (deflating subspaces).


 (If only the generalized eigenvalues are needed, use the driver


 SGGEV instead, which is faster.)


 A generalized eigenvalue for a pair of matrices (A,B) is a scalar w


 or a ratio alpha/beta = w, such that  A - w*B is singular.  It is


 usually represented as the pair (alpha,beta), as there is a


 reasonable interpretation for beta=0 or both being zero.


 A pair of matrices (S,T) is in generalized real Schur form if T is


 upper triangular with non-negative diagonal and S is block upper


 triangular with 1-by-1 and 2-by-2 blocks.  1-by-1 blocks correspond


 to real generalized eigenvalues, while 2-by-2 blocks of S will be


 "standardized" by making the corresponding elements of T have the


 form:


         [  a  0  ]


         [  0  b  ]


 and the pair of corresponding 2-by-2 blocks in S and T will have a


 complex conjugate pair of generalized eigenvalues.

JOBVSL

          JOBVSL is CHARACTER*1


          = 'N':  do not compute the left Schur vectors;


          = 'V':  compute the left Schur vectors.

JOBVSR

          JOBVSR is CHARACTER*1


          = 'N':  do not compute the right Schur vectors;


          = 'V':  compute the right Schur vectors.

SORT

          SORT is CHARACTER*1


          Specifies whether or not to order the eigenvalues on the


          diagonal of the generalized Schur form.


          = 'N':  Eigenvalues are not ordered;


          = 'S':  Eigenvalues are ordered (see SELCTG);

SELCTG

          SELCTG is a LOGICAL FUNCTION of three REAL arguments


          SELCTG must be declared EXTERNAL in the calling subroutine.


          If SORT = 'N', SELCTG is not referenced.


          If SORT = 'S', SELCTG is used to select eigenvalues to sort


          to the top left of the Schur form.


          An eigenvalue (ALPHAR(j)+ALPHAI(j))/BETA(j) is selected if


          SELCTG(ALPHAR(j),ALPHAI(j),BETA(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 in the ill-conditioned case, a selected complex


          eigenvalue may no longer satisfy SELCTG(ALPHAR(j),ALPHAI(j),


          BETA(j)) = .TRUE. after ordering. INFO is to be set to N+2


          in this case.

N

          N is INTEGER


          The order of the matrices A, B, VSL, and VSR.  N >= 0.

A

          A is REAL array, dimension (LDA, N)


          On entry, the first of the pair of matrices.


          On exit, A has been overwritten by its generalized Schur


          form S.

LDA

          LDA is INTEGER


          The leading dimension of A.  LDA >= max(1,N).

B

          B is REAL array, dimension (LDB, N)


          On entry, the second of the pair of matrices.


          On exit, B has been overwritten by its generalized Schur


          form T.

LDB

          LDB is INTEGER


          The leading dimension of B.  LDB >= max(1,N).

SDIM

          SDIM is INTEGER


          If SORT = 'N', SDIM = 0.


          If SORT = 'S', SDIM = number of eigenvalues (after sorting)


          for which SELCTG is true.  (Complex conjugate pairs for which


          SELCTG is true for either eigenvalue count as 2.)

ALPHAR

          ALPHAR is REAL array, dimension (N)

ALPHAI

          ALPHAI is REAL array, dimension (N)

BETA

          BETA is REAL array, dimension (N)


          On exit, (ALPHAR(j) + ALPHAI(j)*i)/BETA(j), j=1,...,N, will


          be the generalized eigenvalues.  ALPHAR(j) + ALPHAI(j)*i,


          and  BETA(j),j=1,...,N are the diagonals of the complex Schur


          form (S,T) that would result if the 2-by-2 diagonal blocks of


          the real Schur form of (A,B) were further reduced to


          triangular form using 2-by-2 complex unitary transformations.


          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) negative.


          Note: the quotients ALPHAR(j)/BETA(j) and ALPHAI(j)/BETA(j)


          may easily over- or underflow, and BETA(j) may even be zero.


          Thus, the user should avoid naively computing the ratio.


          However, ALPHAR and ALPHAI will be always less than and


          usually comparable with norm(A) in magnitude, and BETA always


          less than and usually comparable with norm(B).

VSL

          VSL is REAL array, dimension (LDVSL,N)


          If JOBVSL = 'V', VSL will contain the left Schur vectors.


          Not referenced if JOBVSL = 'N'.

LDVSL

          LDVSL is INTEGER


          The leading dimension of the matrix VSL. LDVSL >=1, and


          if JOBVSL = 'V', LDVSL >= N.

VSR

          VSR is REAL array, dimension (LDVSR,N)


          If JOBVSR = 'V', VSR will contain the right Schur vectors.


          Not referenced if JOBVSR = 'N'.

LDVSR

          LDVSR is INTEGER


          The leading dimension of the matrix VSR. LDVSR >= 1, and


          if JOBVSR = 'V', LDVSR >= N.

WORK

          WORK is REAL array, dimension (MAX(1,LWORK))


          On exit, if INFO = 0, WORK(1) returns the optimal LWORK.

LWORK

          LWORK is INTEGER


          The dimension of the array WORK.


          If N = 0, LWORK >= 1, else LWORK >= max(8*N,6*N+16).


          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.

BWORK

          BWORK is LOGICAL array, dimension (N)


          Not referenced if SORT = 'N'.

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:  =N+1: other than QZ iteration failed in SHGEQZ.


                =N+2: after reordering, roundoff changed values of


                      some complex eigenvalues so that leading


                      eigenvalues in the Generalized Schur form no


                      longer satisfy SELCTG=.TRUE.  This could also


                      be caused due to scaling.


                =N+3: reordering failed in STGSEN.

Univ. of Tennessee

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NAG Ltd.

December 2016