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SUBROUTINE MB04TS( TRANA, TRANB, N, ILO, A, LDA, B, LDB, G, LDG,
$ Q, LDQ, CSL, CSR, TAUL, TAUR, DWORK, LDWORK,
$ INFO )
C
C SLICOT RELEASE 5.0.
C
C Copyright (c) 2002-2009 NICONET e.V.
C
C This program is free software: you can redistribute it and/or
C modify it under the terms of the GNU General Public License as
C published by the Free Software Foundation, either version 2 of
C the License, or (at your option) any later version.
C
C This program is distributed in the hope that it will be useful,
C but WITHOUT ANY WARRANTY; without even the implied warranty of
C MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
C GNU General Public License for more details.
C
C You should have received a copy of the GNU General Public License
C along with this program. If not, see
C <http://www.gnu.org/licenses/>.
C
C PURPOSE
C
C To compute a symplectic URV (SURV) decomposition of a real
C 2N-by-2N matrix H:
C
C [ op(A) G ] T [ op(R11) R12 ] T
C H = [ ] = U R V = U * [ ] * V ,
C [ Q op(B) ] [ 0 op(R22) ]
C
C where A, B, G, Q, R12 are real N-by-N matrices, op(R11) is a real
C N-by-N upper triangular matrix, op(R22) is a real N-by-N lower
C Hessenberg matrix and U, V are 2N-by-2N orthogonal symplectic
C matrices. Unblocked version.
C
C ARGUMENTS
C
C Mode Parameters
C
C TRANA CHARACTER*1
C Specifies the form of op( A ) as follows:
C = 'N': op( A ) = A;
C = 'T': op( A ) = A';
C = 'C': op( A ) = A'.
C
C TRANB CHARACTER*1
C Specifies the form of op( B ) as follows:
C = 'N': op( B ) = B;
C = 'T': op( B ) = B';
C = 'C': op( B ) = B'.
C
C Input/Output Parameters
C
C N (input) INTEGER
C The order of the matrix A. N >= 0.
C
C ILO (input) INTEGER
C It is assumed that op(A) is already upper triangular,
C op(B) is lower triangular and Q is zero in rows and
C columns 1:ILO-1. ILO is normally set by a previous call
C to MB04DD; otherwise it should be set to 1.
C 1 <= ILO <= N, if N > 0; ILO=1, if N=0.
C
C A (input/output) DOUBLE PRECISION array, dimension (LDA,N)
C On entry, the leading N-by-N part of this array must
C contain the matrix A.
C On exit, the leading N-by-N part of this array contains
C the triangular matrix R11, and in the zero part
C information about the elementary reflectors used to
C compute the SURV decomposition.
C
C LDA INTEGER
C The leading dimension of the array A. LDA >= MAX(1,N).
C
C B (input/output) DOUBLE PRECISION array, dimension (LDB,N)
C On entry, the leading N-by-N part of this array must
C contain the matrix B.
C On exit, the leading N-by-N part of this array contains
C the Hessenberg matrix R22, and in the zero part
C information about the elementary reflectors used to
C compute the SURV decomposition.
C
C LDB INTEGER
C The leading dimension of the array B. LDB >= MAX(1,N).
C
C G (input/output) DOUBLE PRECISION array, dimension (LDG,N)
C On entry, the leading N-by-N part of this array must
C contain the matrix G.
C On exit, the leading N-by-N part of this array contains
C the matrix R12.
C
C LDG INTEGER
C The leading dimension of the array G. LDG >= MAX(1,N).
C
C Q (input/output) DOUBLE PRECISION array, dimension (LDQ,N)
C On entry, the leading N-by-N part of this array must
C contain the matrix Q.
C On exit, the leading N-by-N part of this array contains
C information about the elementary reflectors used to
C compute the SURV decomposition.
C
C LDQ INTEGER
C The leading dimension of the array Q. LDG >= MAX(1,N).
C
C CSL (output) DOUBLE PRECISION array, dimension (2N)
C On exit, the first 2N elements of this array contain the
C cosines and sines of the symplectic Givens rotations
C applied from the left-hand side used to compute the SURV
C decomposition.
C
C CSR (output) DOUBLE PRECISION array, dimension (2N-2)
C On exit, the first 2N-2 elements of this array contain the
C cosines and sines of the symplectic Givens rotations
C applied from the right-hand side used to compute the SURV
C decomposition.
C
C TAUL (output) DOUBLE PRECISION array, dimension (N)
C On exit, the first N elements of this array contain the
C scalar factors of some of the elementary reflectors
C applied from the left-hand side.
C
C TAUR (output) DOUBLE PRECISION array, dimension (N-1)
C On exit, the first N-1 elements of this array contain the
C scalar factors of some of the elementary reflectors
C applied from the right-hand side.
C
C Workspace
C
C DWORK DOUBLE PRECISION array, dimension (LDWORK)
C On exit, if INFO = 0, DWORK(1) returns the optimal
C value of LDWORK.
C On exit, if INFO = -16, DWORK(1) returns the minimum
C value of LDWORK.
C
C LDWORK INTEGER
C The length of the array DWORK. LDWORK >= MAX(1,N).
C
C Error Indicator
C
C INFO INTEGER
C = 0: successful exit;
C < 0: if INFO = -i, the i-th argument had an illegal
C value.
C
C METHOD
C
C The matrices U and V are represented as products of symplectic
C reflectors and Givens rotators
C
C U = diag( HU(1),HU(1) ) GU(1) diag( FU(1),FU(1) )
C diag( HU(2),HU(2) ) GU(2) diag( FU(2),FU(2) )
C ....
C diag( HU(n),HU(n) ) GU(n) diag( FU(n),FU(n) ),
C
C V = diag( HV(1),HV(1) ) GV(1) diag( FV(1),FV(1) )
C diag( HV(2),HV(2) ) GV(2) diag( FV(2),FV(2) )
C ....
C diag( HV(n-1),HV(n-1) ) GV(n-1) diag( FV(n-1),FV(n-1) ).
C
C Each HU(i) has the form
C
C HU(i) = I - tau * v * v'
C
C where tau is a real scalar, and v is a real vector with
C v(1:i-1) = 0 and v(i) = 1; v(i+1:n) is stored on exit in
C Q(i+1:n,i), and tau in Q(i,i).
C
C Each FU(i) has the form
C
C FU(i) = I - nu * w * w'
C
C where nu is a real scalar, and w is a real vector with
C w(1:i-1) = 0 and w(i) = 1; w(i+1:n) is stored on exit in
C A(i+1:n,i), if op(A) = 'N', and in A(i,i+1:n), otherwise. The
C scalar nu is stored in TAUL(i).
C
C Each GU(i) is a Givens rotator acting on rows i and n+i,
C where the cosine is stored in CSL(2*i-1) and the sine in
C CSL(2*i).
C
C Each HV(i) has the form
C
C HV(i) = I - tau * v * v'
C
C where tau is a real scalar, and v is a real vector with
C v(1:i) = 0 and v(i+1) = 1; v(i+2:n) is stored on exit in
C Q(i,i+2:n), and tau in Q(i,i+1).
C
C Each FV(i) has the form
C
C FV(i) = I - nu * w * w'
C
C where nu is a real scalar, and w is a real vector with
C w(1:i) = 0 and w(i+1) = 1; w(i+2:n) is stored on exit in
C B(i,i+2:n), if op(B) = 'N', and in B(i+2:n,i), otherwise.
C The scalar nu is stored in TAUR(i).
C
C Each GV(i) is a Givens rotator acting on columns i+1 and n+i+1,
C where the cosine is stored in CSR(2*i-1) and the sine in
C CSR(2*i).
C
C NUMERICAL ASPECTS
C
C The algorithm requires 80/3 N**3 + 20 N**2 + O(N) floating point
C operations and is numerically backward stable.
C
C REFERENCES
C
C [1] Benner, P., Mehrmann, V., and Xu, H.
C A numerically stable, structure preserving method for
C computing the eigenvalues of real Hamiltonian or symplectic
C pencils. Numer. Math., Vol 78 (3), pp. 329-358, 1998.
C
C CONTRIBUTORS
C
C D. Kressner, Technical Univ. Berlin, Germany, and
C P. Benner, Technical Univ. Chemnitz, Germany, December 2003.
C
C REVISIONS
C
C V. Sima, June 2008 (SLICOT version of the HAPACK routine DGESUV).
C
C KEYWORDS
C
C Elementary matrix operations, Matrix decompositions, Hamiltonian
C matrix
C
C ******************************************************************
C
C .. Parameters ..
DOUBLE PRECISION ZERO, ONE
PARAMETER ( ZERO = 0.0D0, ONE = 1.0D0 )
C .. Scalar Arguments ..
CHARACTER TRANA, TRANB
INTEGER ILO, INFO, LDA, LDB, LDG, LDQ, LDWORK, N
C .. Array Arguments ..
DOUBLE PRECISION A(LDA,*), B(LDB,*), CSL(*), CSR(*), DWORK(*),
$ G(LDG,*), Q(LDQ,*), TAUL(*), TAUR(*)
C .. Local Scalars ..
LOGICAL LTRA, LTRB
INTEGER I
DOUBLE PRECISION ALPHA, C, NU, S, TEMP
C .. External Functions ..
LOGICAL LSAME
EXTERNAL LSAME
C .. External Subroutines ..
EXTERNAL DLARF, DLARFG, DLARTG, DROT, XERBLA
C .. Intrinsic Functions ..
INTRINSIC DBLE, MAX
C
C .. Executable Statements ..
C
C Check the scalar input parameters.
C
INFO = 0
LTRA = LSAME( TRANA, 'T' ) .OR. LSAME( TRANA, 'C' )
LTRB = LSAME( TRANB, 'T' ) .OR. LSAME( TRANB, 'C' )
IF ( .NOT.LTRA .AND. .NOT.LSAME( TRANA, 'N' ) ) THEN
INFO = -1
ELSE IF ( .NOT.LTRB .AND. .NOT.LSAME( TRANB, 'N' ) ) THEN
INFO = -2
ELSE IF ( N.LT.0 ) THEN
INFO = -3
ELSE IF ( ILO.LT.1 .OR. ILO.GT.MAX( 1, N ) ) THEN
INFO = -4
ELSE IF ( LDA.LT.MAX( 1, N ) ) THEN
INFO = -6
ELSE IF ( LDB.LT.MAX( 1, N ) ) THEN
INFO = -8
ELSE IF ( LDG.LT.MAX( 1, N ) ) THEN
INFO = -10
ELSE IF ( LDQ.LT.MAX( 1, N ) ) THEN
INFO = -12
ELSE IF ( LDWORK.LT.MAX( 1, N ) ) THEN
DWORK(1) = DBLE( MAX( 1, N ) )
INFO = -18
END IF
C
C Return if there were illegal values.
C
IF ( INFO.NE.0 ) THEN
CALL XERBLA( 'MB04TS', -INFO )
RETURN
END IF
C
C Quick return if possible.
C
IF ( N.EQ.0 ) THEN
DWORK(1) = ONE
RETURN
END IF
C
DO 10 I = ILO, N
ALPHA = Q(I,I)
IF ( I.LT.N ) THEN
C
C Generate elementary reflector HU(i) to annihilate Q(i+1:n,i)
C
CALL DLARFG( N-I+1, ALPHA, Q(I+1,I), 1, NU )
C
C Apply HU(i) from the left.
C
Q(I,I) = ONE
CALL DLARF( 'Left', N-I+1, N-I, Q(I,I), 1, NU, Q(I,I+1),
$ LDQ, DWORK )
IF ( LTRA ) THEN
CALL DLARF( 'Right', N-I+1, N-I+1, Q(I,I), 1, NU, A(I,I),
$ LDA, DWORK )
ELSE
CALL DLARF( 'Left', N-I+1, N-I+1, Q(I,I), 1, NU, A(I,I),
$ LDA, DWORK )
END IF
IF ( LTRB ) THEN
CALL DLARF( 'Right', N, N-I+1, Q(I,I), 1, NU, B(1,I),
$ LDB, DWORK )
ELSE
CALL DLARF( 'Left', N-I+1, N, Q(I,I), 1, NU, B(I,1), LDB,
$ DWORK )
END IF
CALL DLARF( 'Left', N-I+1, N, Q(I,I), 1, NU, G(I,1), LDG,
$ DWORK )
Q(I,I) = NU
ELSE
Q(I,I) = ZERO
END IF
C
C Generate symplectic Givens rotator GU(i) to annihilate Q(i,i).
C
TEMP = A(I,I)
CALL DLARTG( TEMP, ALPHA, C, S, A(I,I) )
C
C Apply G(i) from the left.
C
IF ( LTRA ) THEN
CALL DROT( N-I, A(I+1,I), 1, Q(I,I+1), LDQ, C, S )
ELSE
CALL DROT( N-I, A(I,I+1), LDA, Q(I,I+1), LDQ, C, S )
END IF
IF ( LTRB ) THEN
CALL DROT( N, G(I,1), LDG, B(1,I), 1, C, S )
ELSE
CALL DROT( N, G(I,1), LDG, B(I,1), LDB, C, S )
END IF
CSL(2*I-1) = C
CSL(2*I) = S
C
IF ( I.LT.N ) THEN
IF ( LTRA ) THEN
C
C Generate elementary reflector FU(i) to annihilate
C A(i,i+1:n).
C
CALL DLARFG( N-I+1, A(I,I), A(I,I+1), LDA, TAUL(I) )
C
C Apply FU(i) from the left.
C
TEMP = A(I,I)
A(I,I) = ONE
CALL DLARF( 'Right', N-I, N-I+1, A(I,I), LDA, TAUL(I),
$ A(I+1,I), LDA, DWORK )
CALL DLARF( 'Left', N-I+1, N-I, A(I,I), LDA, TAUL(I),
$ Q(I,I+1), LDQ, DWORK )
IF ( LTRB ) THEN
CALL DLARF( 'Right', N, N-I+1, A(I,I), LDA, TAUL(I),
$ B(1,I), LDB, DWORK )
ELSE
CALL DLARF( 'Left', N-I+1, N, A(I,I), LDA, TAUL(I),
$ B(I,1), LDB, DWORK )
END IF
CALL DLARF( 'Left', N-I+1, N, A(I,I), LDA, TAUL(I),
$ G(I,1), LDG, DWORK )
A(I,I) = TEMP
ELSE
C
C Generate elementary reflector FU(i) to annihilate
C A(i+1:n,i).
C
CALL DLARFG( N-I+1, A(I,I), A(I+1,I), 1, TAUL(I) )
C
C Apply FU(i) from the left.
C
TEMP = A(I,I)
A(I,I) = ONE
CALL DLARF( 'Left', N-I+1, N-I, A(I,I), 1, TAUL(I),
$ A(I,I+1), LDA, DWORK )
CALL DLARF( 'Left', N-I+1, N-I, A(I,I), 1, TAUL(I),
$ Q(I,I+1), LDQ, DWORK )
IF ( LTRB ) THEN
CALL DLARF( 'Right', N, N-I+1, A(I,I), 1, TAUL(I),
$ B(1,I), LDB, DWORK )
ELSE
CALL DLARF( 'Left', N-I+1, N, A(I,I), 1, TAUL(I),
$ B(I,1), LDB, DWORK )
END IF
CALL DLARF( 'Left', N-I+1, N, A(I,I), 1, TAUL(I), G(I,1),
$ LDG, DWORK )
A(I,I) = TEMP
END IF
ELSE
TAUL(I) = ZERO
END IF
IF ( I.LT.N )
$ ALPHA = Q(I,I+1)
IF ( I.LT.N-1 ) THEN
C
C Generate elementary reflector HV(i) to annihilate Q(i,i+2:n)
C
CALL DLARFG( N-I, ALPHA, Q(I,I+2), LDQ, NU )
C
C Apply HV(i) from the right.
C
Q(I,I+1) = ONE
CALL DLARF( 'Right', N-I, N-I, Q(I,I+1), LDQ, NU,
$ Q(I+1,I+1), LDQ, DWORK )
IF ( LTRA ) THEN
CALL DLARF( 'Left', N-I, N, Q(I,I+1), LDQ, NU,
$ A(I+1,1), LDA, DWORK )
ELSE
CALL DLARF( 'Right', N, N-I, Q(I,I+1), LDQ, NU,
$ A(1,I+1), LDA, DWORK )
END IF
IF ( LTRB ) THEN
CALL DLARF( 'Left', N-I, N-I+1, Q(I,I+1), LDQ, NU,
$ B(I+1,I), LDB, DWORK )
ELSE
CALL DLARF( 'Right', N-I+1, N-I, Q(I,I+1), LDQ, NU,
$ B(I,I+1), LDB, DWORK )
END IF
CALL DLARF( 'Right', N, N-I, Q(I,I+1), LDQ, NU,
$ G(1,I+1), LDG, DWORK )
Q(I,I+1) = NU
ELSE IF ( I.LT.N ) THEN
Q(I,I+1) = ZERO
END IF
IF ( I.LT.N ) THEN
C
C Generate symplectic Givens rotator GV(i) to annihilate
C Q(i,i+1).
C
IF ( LTRB ) THEN
TEMP = B(I+1,I)
CALL DLARTG( TEMP, ALPHA, C, S, B(I+1,I) )
S = -S
CALL DROT( N-I, Q(I+1,I+1), 1, B(I+1,I+1), LDB, C, S )
ELSE
TEMP = B(I,I+1)
CALL DLARTG( TEMP, ALPHA, C, S, B(I,I+1) )
S = -S
CALL DROT( N-I, Q(I+1,I+1), 1, B(I+1,I+1), 1, C, S )
END IF
IF ( LTRA ) THEN
CALL DROT( N, A(I+1,1), LDA, G(1,I+1), 1, C, S )
ELSE
CALL DROT( N, A(1,I+1), 1, G(1,I+1), 1, C, S )
END IF
CSR(2*I-1) = C
CSR(2*I) = S
END IF
IF ( I.LT.N-1 ) THEN
IF ( LTRB ) THEN
C
C Generate elementary reflector FV(i) to annihilate
C B(i+2:n,i).
C
CALL DLARFG( N-I, B(I+1,I), B(I+2,I), 1, TAUR(I) )
C
C Apply FV(i) from the right.
C
TEMP = B(I+1,I)
B(I+1,I) = ONE
CALL DLARF( 'Left', N-I, N-I, B(I+1,I), 1, TAUR(I),
$ B(I+1,I+1), LDB, DWORK )
CALL DLARF( 'Right', N-I, N-I, B(I+1,I), 1, TAUR(I),
$ Q(I+1,I+1), LDQ, DWORK )
IF ( LTRA ) THEN
CALL DLARF( 'Left', N-I, N, B(I+1,I), 1,
$ TAUR(I), A(I+1,1), LDA, DWORK )
ELSE
CALL DLARF( 'Right', N, N-I, B(I+1,I), 1,
$ TAUR(I), A(1,I+1), LDA, DWORK )
END IF
CALL DLARF( 'Right', N, N-I, B(I+1,I), 1, TAUR(I),
$ G(1,I+1), LDG, DWORK )
B(I+1,I) = TEMP
ELSE
C
C Generate elementary reflector FV(i) to annihilate
C B(i,i+2:n).
C
CALL DLARFG( N-I, B(I,I+1), B(I,I+2), LDB, TAUR(I) )
C
C Apply FV(i) from the right.
C
TEMP = B(I,I+1)
B(I,I+1) = ONE
CALL DLARF( 'Right', N-I, N-I, B(I,I+1), LDB, TAUR(I),
$ B(I+1,I+1), LDB, DWORK )
CALL DLARF( 'Right', N-I, N-I, B(I,I+1), LDB, TAUR(I),
$ Q(I+1,I+1), LDQ, DWORK )
IF ( LTRA ) THEN
CALL DLARF( 'Left', N-I, N, B(I,I+1), LDB, TAUR(I),
$ A(I+1,1), LDA, DWORK )
ELSE
CALL DLARF( 'Right', N, N-I, B(I,I+1), LDB,
$ TAUR(I), A(1,I+1), LDA, DWORK )
END IF
CALL DLARF( 'Right', N, N-I, B(I,I+1), LDB, TAUR(I),
$ G(1,I+1), LDG, DWORK )
B(I,I+1) = TEMP
END IF
ELSE IF ( I.LT.N ) THEN
TAUR(I) = ZERO
END IF
10 CONTINUE
DWORK(1) = DBLE( MAX( 1, N ) )
RETURN
C *** Last line of MB04TS ***
END
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