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SGEGS(3F)							     SGEGS(3F)


NAME    [Toc]    [Back]

     SGEGS - compute for a pair	of N-by-N real nonsymmetric matrices A,	B

SYNOPSIS    [Toc]    [Back]

     SUBROUTINE	SGEGS( JOBVSL, JOBVSR, N, A, LDA, B, LDB, ALPHAR, ALPHAI,
		       BETA, VSL, LDVSL, VSR, LDVSR, WORK, LWORK, INFO )

	 CHARACTER     JOBVSL, JOBVSR

	 INTEGER       INFO, LDA, LDB, LDVSL, LDVSR, LWORK, N

	 REAL	       A( LDA, * ), ALPHAI( * ), ALPHAR( * ), B( LDB, *	),
		       BETA( * ), VSL( LDVSL, *	), VSR(	LDVSR, * ), WORK( * )

PURPOSE    [Toc]    [Back]

     SGEGS computes for	a pair of N-by-N real nonsymmetric matrices A, B:  the
     generalized eigenvalues (alphar +/- alphai*i, beta), the real Schur form
     (A, B), and optionally left and/or	right Schur vectors (VSL and VSR).

     (If only the generalized eigenvalues are needed, use the driver SGEGV
     instead.)

     A generalized eigenvalue for a pair of matrices (A,B) is, roughly
     speaking, 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,	and even for both being	zero.
     A good beginning reference	is the book, "Matrix Computations", by G.
     Golub & C.	van Loan (Johns	Hopkins	U. Press)

     The (generalized) Schur form of a pair of matrices	is the result of
     multiplying both matrices on the left by one orthogonal matrix and	both
     on	the right by another orthogonal	matrix,	these two orthogonal matrices
     being chosen so as	to bring the pair of matrices into (real) Schur	form.

     A pair of matrices	A, B is	in generalized real Schur form if B is upper
     triangular	with non-negative diagonal and A 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 A will be "standardized"
     by	making the corresponding elements of B have the	form:
	     [	a  0  ]
	     [	0  b  ]

     and the pair of corresponding 2-by-2 blocks in A and B will have a
     complex conjugate pair of generalized eigenvalues.

     The left and right	Schur vectors are the columns of VSL and VSR,
     respectively, where VSL and VSR are the orthogonal	matrices which reduce
     A and B to	Schur form:

     Schur form	of (A,B) = ( (VSL)**T A	(VSR), (VSL)**T	B (VSR)	)





									Page 1






SGEGS(3F)							     SGEGS(3F)


ARGUMENTS    [Toc]    [Back]

     JOBVSL  (input) CHARACTER*1
	     = 'N':  do	not compute the	left Schur vectors;
	     = 'V':  compute the left Schur vectors.

     JOBVSR  (input) CHARACTER*1
	     = 'N':  do	not compute the	right Schur vectors;
	     = 'V':  compute the right Schur vectors.

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

     A	     (input/output) REAL array,	dimension (LDA,	N)
	     On	entry, the first of the	pair of	matrices whose generalized
	     eigenvalues and (optionally) Schur	vectors	are to be computed.
	     On	exit, the generalized Schur form of A.	Note: to avoid
	     overflow, the Frobenius norm of the matrix	A should be less than
	     the overflow threshold.

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

     B	     (input/output) REAL array,	dimension (LDB,	N)
	     On	entry, the second of the pair of matrices whose	generalized
	     eigenvalues and (optionally) Schur	vectors	are to be computed.
	     On	exit, the generalized Schur form of B.	Note: to avoid
	     overflow, the Frobenius norm of the matrix	B should be less than
	     the overflow threshold.

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

     ALPHAR  (output) REAL array, dimension (N)
	     ALPHAI  (output) REAL array, dimension (N)	BETA	(output) 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, j=1,...,N  and  BETA(j),j=1,...,N  are the diagonals
	     of	the complex Schur form (A,B) 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 alpha/beta.
	     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).





									Page 2






SGEGS(3F)							     SGEGS(3F)



     VSL     (output) REAL array, dimension (LDVSL,N)
	     If	JOBVSL = 'V', VSL will contain the left	Schur vectors.	(See
	     "Purpose",	above.)	 Not referenced	if JOBVSL = 'N'.

     LDVSL   (input) INTEGER
	     The leading dimension of the matrix VSL. LDVSL >=1, and if	JOBVSL
	     = 'V', LDVSL >= N.

     VSR     (output) REAL array, dimension (LDVSR,N)
	     If	JOBVSR = 'V', VSR will contain the right Schur vectors.	 (See
	     "Purpose",	above.)	 Not referenced	if JOBVSR = 'N'.

     LDVSR   (input) INTEGER
	     The leading dimension of the matrix VSR. LDVSR >= 1, and if
	     JOBVSR = 'V', LDVSR >= N.

     WORK    (workspace/output)	REAL array, dimension (LWORK)
	     On	exit, if INFO =	0, WORK(1) returns the optimal LWORK.

     LWORK   (input) 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).

     INFO    (output) 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)
SGEGS(3F)							     SGEGS(3F)


NAME    [Toc]    [Back]

     SGEGS - compute for a pair	of N-by-N real nonsymmetric matrices A,	B

SYNOPSIS    [Toc]    [Back]

     SUBROUTINE	SGEGS( JOBVSL, JOBVSR, N, A, LDA, B, LDB, ALPHAR, ALPHAI,
		       BETA, VSL, LDVSL, VSR, LDVSR, WORK, LWORK, INFO )

	 CHARACTER     JOBVSL, JOBVSR

	 INTEGER       INFO, LDA, LDB, LDVSL, LDVSR, LWORK, N

	 REAL	       A( LDA, * ), ALPHAI( * ), ALPHAR( * ), B( LDB, *	),
		       BETA( * ), VSL( LDVSL, *	), VSR(	LDVSR, * ), WORK( * )

PURPOSE    [Toc]    [Back]

     SGEGS computes for	a pair of N-by-N real nonsymmetric matrices A, B:  the
     generalized eigenvalues (alphar +/- alphai*i, beta), the real Schur form
     (A, B), and optionally left and/or	right Schur vectors (VSL and VSR).

     (If only the generalized eigenvalues are needed, use the driver SGEGV
     instead.)

     A generalized eigenvalue for a pair of matrices (A,B) is, roughly
     speaking, 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,	and even for both being	zero.
     A good beginning reference	is the book, "Matrix Computations", by G.
     Golub & C.	van Loan (Johns	Hopkins	U. Press)

     The (generalized) Schur form of a pair of matrices	is the result of
     multiplying both matrices on the left by one orthogonal matrix and	both
     on	the right by another orthogonal	matrix,	these two orthogonal matrices
     being chosen so as	to bring the pair of matrices into (real) Schur	form.

     A pair of matrices	A, B is	in generalized real Schur form if B is upper
     triangular	with non-negative diagonal and A 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 A will be "standardized"
     by	making the corresponding elements of B have the	form:
	     [	a  0  ]
	     [	0  b  ]

     and the pair of corresponding 2-by-2 blocks in A and B will have a
     complex conjugate pair of generalized eigenvalues.

     The left and right	Schur vectors are the columns of VSL and VSR,
     respectively, where VSL and VSR are the orthogonal	matrices which reduce
     A and B to	Schur form:

     Schur form	of (A,B) = ( (VSL)**T A	(VSR), (VSL)**T	B (VSR)	)





									Page 1






SGEGS(3F)							     SGEGS(3F)


ARGUMENTS    [Toc]    [Back]

     JOBVSL  (input) CHARACTER*1
	     = 'N':  do	not compute the	left Schur vectors;
	     = 'V':  compute the left Schur vectors.

     JOBVSR  (input) CHARACTER*1
	     = 'N':  do	not compute the	right Schur vectors;
	     = 'V':  compute the right Schur vectors.

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

     A	     (input/output) REAL array,	dimension (LDA,	N)
	     On	entry, the first of the	pair of	matrices whose generalized
	     eigenvalues and (optionally) Schur	vectors	are to be computed.
	     On	exit, the generalized Schur form of A.	Note: to avoid
	     overflow, the Frobenius norm of the matrix	A should be less than
	     the overflow threshold.

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

     B	     (input/output) REAL array,	dimension (LDB,	N)
	     On	entry, the second of the pair of matrices whose	generalized
	     eigenvalues and (optionally) Schur	vectors	are to be computed.
	     On	exit, the generalized Schur form of B.	Note: to avoid
	     overflow, the Frobenius norm of the matrix	B should be less than
	     the overflow threshold.

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

     ALPHAR  (output) REAL array, dimension (N)
	     ALPHAI  (output) REAL array, dimension (N)	BETA	(output) 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, j=1,...,N  and  BETA(j),j=1,...,N  are the diagonals
	     of	the complex Schur form (A,B) 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 alpha/beta.
	     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).





									Page 2






SGEGS(3F)							     SGEGS(3F)



     VSL     (output) REAL array, dimension (LDVSL,N)
	     If	JOBVSL = 'V', VSL will contain the left	Schur vectors.	(See
	     "Purpose",	above.)	 Not referenced	if JOBVSL = 'N'.

     LDVSL   (input) INTEGER
	     The leading dimension of the matrix VSL. LDVSL >=1, and if	JOBVSL
	     = 'V', LDVSL >= N.

     VSR     (output) REAL array, dimension (LDVSR,N)
	     If	JOBVSR = 'V', VSR will contain the right Schur vectors.	 (See
	     "Purpose",	above.)	 Not referenced	if JOBVSR = 'N'.

     LDVSR   (input) INTEGER
	     The leading dimension of the matrix VSR. LDVSR >= 1, and if
	     JOBVSR = 'V', LDVSR >= N.

     WORK    (workspace/output)	REAL array, dimension (LWORK)
	     On	exit, if INFO =	0, WORK(1) returns the optimal LWORK.

     LWORK   (input) 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).

     INFO    (output) 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)


									PPPPaaaaggggeeee 3333
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