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ctgsy2.f(3)				      LAPACK				      ctgsy2.f(3)

NAME
       ctgsy2.f -

SYNOPSIS
   Functions/Subroutines
       subroutine ctgsy2 (TRANS, IJOB, M, N, A, LDA, B, LDB, C, LDC, D, LDD, E, LDE, F, LDF,
	   SCALE, RDSUM, RDSCAL, INFO)
	   CTGSY2 solves the generalized Sylvester equation (unblocked algorithm).

Function/Subroutine Documentation
   subroutine ctgsy2 (characterTRANS, integerIJOB, integerM, integerN, complex, dimension( lda, *
       )A, integerLDA, complex, dimension( ldb, * )B, integerLDB, complex, dimension( ldc, * )C,
       integerLDC, complex, dimension( ldd, * )D, integerLDD, complex, dimension( lde, * )E,
       integerLDE, complex, dimension( ldf, * )F, integerLDF, realSCALE, realRDSUM, realRDSCAL,
       integerINFO)
       CTGSY2 solves the generalized Sylvester equation (unblocked algorithm).

       Purpose:

	    CTGSY2 solves the generalized Sylvester equation

			A * R - L * B = scale *  C		 (1)
			D * R - L * E = scale * F

	    using Level 1 and 2 BLAS, where R and L are unknown M-by-N matrices,
	    (A, D), (B, E) and (C, F) are given matrix pairs of size M-by-M,
	    N-by-N and M-by-N, respectively. A, B, D and E are upper triangular
	    (i.e., (A,D) and (B,E) in generalized Schur form).

	    The solution (R, L) overwrites (C, F). 0 <= SCALE <= 1 is an output
	    scaling factor chosen to avoid overflow.

	    In matrix notation solving equation (1) corresponds to solve
	    Zx = scale * b, where Z is defined as

		   Z = [ kron(In, A)  -kron(B**H, Im) ] 	    (2)
		       [ kron(In, D)  -kron(E**H, Im) ],

	    Ik is the identity matrix of size k and X**H is the transpose of X.
	    kron(X, Y) is the Kronecker product between the matrices X and Y.

	    If TRANS = 'C', y in the conjugate transposed system Z**H*y = scale*b
	    is solved for, which is equivalent to solve for R and L in

			A**H * R  + D**H * L   = scale * C	     (3)
			R  * B**H + L  * E**H  = scale * -F

	    This case is used to compute an estimate of Dif[(A, D), (B, E)] =
	    = sigma_min(Z) using reverse communicaton with CLACON.

	    CTGSY2 also (IJOB >= 1) contributes to the computation in CTGSYL
	    of an upper bound on the separation between to matrix pairs. Then
	    the input (A, D), (B, E) are sub-pencils of two matrix pairs in
	    CTGSYL.

       Parameters:
	   TRANS

		     TRANS is CHARACTER*1
		     = 'N', solve the generalized Sylvester equation (1).
		     = 'T': solve the 'transposed' system (3).

	   IJOB

		     IJOB is INTEGER
		     Specifies what kind of functionality to be performed.
		     =0: solve (1) only.
		     =1: A contribution from this subsystem to a Frobenius
			 norm-based estimate of the separation between two matrix
			 pairs is computed. (look ahead strategy is used).
		     =2: A contribution from this subsystem to a Frobenius
			 norm-based estimate of the separation between two matrix
			 pairs is computed. (SGECON on sub-systems is used.)
		     Not referenced if TRANS = 'T'.

	   M

		     M is INTEGER
		     On entry, M specifies the order of A and D, and the row
		     dimension of C, F, R and L.

	   N

		     N is INTEGER
		     On entry, N specifies the order of B and E, and the column
		     dimension of C, F, R and L.

	   A

		     A is COMPLEX array, dimension (LDA, M)
		     On entry, A contains an upper triangular matrix.

	   LDA

		     LDA is INTEGER
		     The leading dimension of the matrix A. LDA >= max(1, M).

	   B

		     B is COMPLEX array, dimension (LDB, N)
		     On entry, B contains an upper triangular matrix.

	   LDB

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

	   C

		     C is COMPLEX array, dimension (LDC, N)
		     On entry, C contains the right-hand-side of the first matrix
		     equation in (1).
		     On exit, if IJOB = 0, C has been overwritten by the solution
		     R.

	   LDC

		     LDC is INTEGER
		     The leading dimension of the matrix C. LDC >= max(1, M).

	   D

		     D is COMPLEX array, dimension (LDD, M)
		     On entry, D contains an upper triangular matrix.

	   LDD

		     LDD is INTEGER
		     The leading dimension of the matrix D. LDD >= max(1, M).

	   E

		     E is COMPLEX array, dimension (LDE, N)
		     On entry, E contains an upper triangular matrix.

	   LDE

		     LDE is INTEGER
		     The leading dimension of the matrix E. LDE >= max(1, N).

	   F

		     F is COMPLEX array, dimension (LDF, N)
		     On entry, F contains the right-hand-side of the second matrix
		     equation in (1).
		     On exit, if IJOB = 0, F has been overwritten by the solution
		     L.

	   LDF

		     LDF is INTEGER
		     The leading dimension of the matrix F. LDF >= max(1, M).

	   SCALE

		     SCALE is REAL
		     On exit, 0 <= SCALE <= 1. If 0 < SCALE < 1, the solutions
		     R and L (C and F on entry) will hold the solutions to a
		     slightly perturbed system but the input matrices A, B, D and
		     E have not been changed. If SCALE = 0, R and L will hold the
		     solutions to the homogeneous system with C = F = 0.
		     Normally, SCALE = 1.

	   RDSUM

		     RDSUM is REAL
		     On entry, the sum of squares of computed contributions to
		     the Dif-estimate under computation by CTGSYL, where the
		     scaling factor RDSCAL (see below) has been factored out.
		     On exit, the corresponding sum of squares updated with the
		     contributions from the current sub-system.
		     If TRANS = 'T' RDSUM is not touched.
		     NOTE: RDSUM only makes sense when CTGSY2 is called by
		     CTGSYL.

	   RDSCAL

		     RDSCAL is REAL
		     On entry, scaling factor used to prevent overflow in RDSUM.
		     On exit, RDSCAL is updated w.r.t. the current contributions
		     in RDSUM.
		     If TRANS = 'T', RDSCAL is not touched.
		     NOTE: RDSCAL only makes sense when CTGSY2 is called by
		     CTGSYL.

	   INFO

		     INFO is INTEGER
		     On exit, if INFO is set to
		       =0: Successful exit
		       <0: If INFO = -i, input argument number i is illegal.
		       >0: The matrix pairs (A, D) and (B, E) have common or very
			   close eigenvalues.

       Author:
	   Univ. of Tennessee

	   Univ. of California Berkeley

	   Univ. of Colorado Denver

	   NAG Ltd.

       Date:
	   September 2012

       Contributors:
	   Bo Kagstrom and Peter Poromaa, Department of Computing Science, Umea University, S-901
	   87 Umea, Sweden.

       Definition at line 258 of file ctgsy2.f.

Author
       Generated automatically by Doxygen for LAPACK from the source code.

Version 3.4.2				 Tue Sep 25 2012			      ctgsy2.f(3)
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