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SUBROUTINE AB01MD( JOBZ, N, A, LDA, B, NCONT, Z, LDZ, TAU, TOL,
$ 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 find a controllable realization for the linear time-invariant
C single-input system
C
C dX/dt = A * X + B * U,
C
C where A is an N-by-N matrix and B is an N element vector which
C are reduced by this routine to orthogonal canonical form using
C (and optionally accumulating) orthogonal similarity
C transformations.
C
C ARGUMENTS
C
C Mode Parameters
C
C JOBZ CHARACTER*1
C Indicates whether the user wishes to accumulate in a
C matrix Z the orthogonal similarity transformations for
C reducing the system, as follows:
C = 'N': Do not form Z and do not store the orthogonal
C transformations;
C = 'F': Do not form Z, but store the orthogonal
C transformations in the factored form;
C = 'I': Z is initialized to the unit matrix and the
C orthogonal transformation matrix Z is returned.
C
C Input/Output Parameters
C
C N (input) INTEGER
C The order of the original state-space representation,
C i.e. the order of the matrix A. 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 original state dynamics matrix A.
C On exit, the leading NCONT-by-NCONT upper Hessenberg
C part of this array contains the canonical form of the
C state dynamics matrix, given by Z' * A * Z, of a
C controllable realization for the original system. The
C elements below the first subdiagonal are set to zero.
C
C LDA INTEGER
C The leading dimension of array A. LDA >= MAX(1,N).
C
C B (input/output) DOUBLE PRECISION array, dimension (N)
C On entry, the original input/state vector B.
C On exit, the leading NCONT elements of this array contain
C canonical form of the input/state vector, given by Z' * B,
C with all elements but B(1) set to zero.
C
C NCONT (output) INTEGER
C The order of the controllable state-space representation.
C
C Z (output) DOUBLE PRECISION array, dimension (LDZ,N)
C If JOBZ = 'I', then the leading N-by-N part of this array
C contains the matrix of accumulated orthogonal similarity
C transformations which reduces the given system to
C orthogonal canonical form.
C If JOBZ = 'F', the elements below the diagonal, with the
C array TAU, represent the orthogonal transformation matrix
C as a product of elementary reflectors. The transformation
C matrix can then be obtained by calling the LAPACK Library
C routine DORGQR.
C If JOBZ = 'N', the array Z is not referenced and can be
C supplied as a dummy array (i.e. set parameter LDZ = 1 and
C declare this array to be Z(1,1) in the calling program).
C
C LDZ INTEGER
C The leading dimension of array Z. If JOBZ = 'I' or
C JOBZ = 'F', LDZ >= MAX(1,N); if JOBZ = 'N', LDZ >= 1.
C
C TAU (output) DOUBLE PRECISION array, dimension (N)
C The elements of TAU contain the scalar factors of the
C elementary reflectors used in the reduction of B and A.
C
C Tolerances
C
C TOL DOUBLE PRECISION
C The tolerance to be used in determining the
C controllability of (A,B). If the user sets TOL > 0, then
C the given value of TOL is used as an absolute tolerance;
C elements with absolute value less than TOL are considered
C neglijible. If the user sets TOL <= 0, then an implicitly
C computed, default tolerance, defined by
C TOLDEF = N*EPS*MAX( NORM(A), NORM(B) ) is used instead,
C where EPS is the machine precision (see LAPACK Library
C routine DLAMCH).
C
C Workspace
C
C DWORK DOUBLE PRECISION array, dimension (LDWORK)
C On exit, if INFO = 0, DWORK(1) returns the optimal value
C of LDWORK.
C
C LDWORK INTEGER
C The length of the array DWORK. LDWORK >= MAX(1,N).
C For optimum performance LDWORK should be larger.
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 Householder matrix which reduces all but the first element
C of vector B to zero is found and this orthogonal similarity
C transformation is applied to the matrix A. The resulting A is then
C reduced to upper Hessenberg form by a sequence of Householder
C transformations. Finally, the order of the controllable state-
C space representation (NCONT) is determined by finding the position
C of the first sub-diagonal element of A which is below an
C appropriate zero threshold, either TOL or TOLDEF (see parameter
C TOL); if NORM(B) is smaller than this threshold, NCONT is set to
C zero, and no computations for reducing the system to orthogonal
C canonical form are performed.
C
C REFERENCES
C
C [1] Konstantinov, M.M., Petkov, P.Hr. and Christov, N.D.
C Orthogonal Invariants and Canonical Forms for Linear
C Controllable Systems.
C Proc. 8th IFAC World Congress, Kyoto, 1, pp. 49-54, 1981.
C
C [2] Hammarling, S.J.
C Notes on the use of orthogonal similarity transformations in
C control.
C NPL Report DITC 8/82, August 1982.
C
C [3] Paige, C.C
C Properties of numerical algorithms related to computing
C controllability.
C IEEE Trans. Auto. Contr., AC-26, pp. 130-138, 1981.
C
C NUMERICAL ASPECTS
C 3
C The algorithm requires 0(N ) operations and is backward stable.
C
C CONTRIBUTOR
C
C Release 3.0: V. Sima, Katholieke Univ. Leuven, Belgium, Oct. 1996.
C Supersedes Release 2.0 routine AB01AD by T.W.C. Williams,
C Kingston Polytechnic, United Kingdom, October 1982.
C
C REVISIONS
C
C V. Sima, February 16, 1998, October 19, 2001, February 2, 2005.
C
C KEYWORDS
C
C Controllability, minimal realization, orthogonal canonical form,
C orthogonal transformation.
C
C ******************************************************************
C
C .. Parameters ..
DOUBLE PRECISION ZERO, ONE
PARAMETER ( ZERO = 0.0D0, ONE = 1.0D0 )
C .. Scalar Arguments ..
CHARACTER JOBZ
INTEGER INFO, LDA, LDZ, LDWORK, N, NCONT
DOUBLE PRECISION TOL
C .. Array Arguments ..
DOUBLE PRECISION A(LDA,*), B(*), DWORK(*), TAU(*), Z(LDZ,*)
C .. Local Scalars ..
LOGICAL LJOBF, LJOBI, LJOBZ
INTEGER ITAU, J
DOUBLE PRECISION ANORM, B1, BNORM, FANORM, FBNORM, H, THRESH,
$ TOLDEF, WRKOPT
C .. Local Arrays ..
DOUBLE PRECISION NBLK(1)
C .. External Functions ..
LOGICAL LSAME
DOUBLE PRECISION DLAMCH, DLANGE
EXTERNAL DLAMCH, DLANGE, LSAME
C .. External Subroutines ..
EXTERNAL DGEHRD, DLACPY, DLARF, DLARFG, DLASET, DORGQR,
$ MB01PD, XERBLA
C .. Intrinsic Functions ..
INTRINSIC ABS, DBLE, MAX
C .. Executable Statements ..
C
INFO = 0
LJOBF = LSAME( JOBZ, 'F' )
LJOBI = LSAME( JOBZ, 'I' )
LJOBZ = LJOBF.OR.LJOBI
C
C Test the input scalar arguments.
C
IF( .NOT.LJOBZ .AND. .NOT.LSAME( JOBZ, 'N' ) ) THEN
INFO = -1
ELSE IF( N.LT.0 ) THEN
INFO = -2
ELSE IF( LDA.LT.MAX(1,N) ) THEN
INFO = -4
ELSE IF( .NOT.LJOBZ .AND. LDZ.LT.1 .OR.
$ LJOBZ .AND. LDZ.LT.MAX(1,N) ) THEN
INFO = -8
ELSE IF( LDWORK.LT.MAX( 1, N ) ) THEN
INFO = -12
END IF
C
IF ( INFO.NE.0 ) THEN
C
C Error return.
C
CALL XERBLA( 'AB01MD', -INFO )
RETURN
END IF
C
C Quick return if possible.
C
NCONT = 0
DWORK(1) = ONE
IF ( N.EQ.0 )
$ RETURN
C
C (Note: Comments in the code beginning "Workspace:" describe the
C minimal amount of real workspace needed at that point in the
C code, as well as the preferred amount for good performance.
C NB refers to the optimal block size for the immediately
C following subroutine, as returned by ILAENV.)
C
WRKOPT = ONE
C
C Calculate the absolute norms of A and B (used for scaling).
C
ANORM = DLANGE( 'M', N, N, A, LDA, DWORK )
BNORM = DLANGE( 'M', N, 1, B, N, DWORK )
C
C Return if matrix B is zero.
C
IF( BNORM.EQ.ZERO ) THEN
IF( LJOBF ) THEN
CALL DLASET( 'F', N, N, ZERO, ZERO, Z, LDZ )
CALL DLASET( 'F', N, 1, ZERO, ZERO, TAU, N )
ELSE IF( LJOBI ) THEN
CALL DLASET( 'F', N, N, ZERO, ONE, Z, LDZ )
END IF
RETURN
END IF
C
C Scale (if needed) the matrices A and B.
C
CALL MB01PD( 'S', 'G', N, N, 0, 0, ANORM, 0, NBLK, A, LDA, INFO )
CALL MB01PD( 'S', 'G', N, 1, 0, 0, BNORM, 0, NBLK, B, N, INFO )
C
C Calculate the Frobenius norm of A and the 1-norm of B (used for
C controlability test).
C
FANORM = DLANGE( 'F', N, N, A, LDA, DWORK )
FBNORM = DLANGE( '1', N, 1, B, N, DWORK )
C
TOLDEF = TOL
IF ( TOLDEF.LE.ZERO ) THEN
C
C Use the default tolerance in controllability determination.
C
THRESH = DBLE(N)*DLAMCH( 'EPSILON' )
TOLDEF = THRESH*MAX( FANORM, FBNORM )
END IF
C
ITAU = 1
IF ( FBNORM.GT.TOLDEF ) THEN
C
C B is not negligible compared with A.
C
IF ( N.GT.1 ) THEN
C
C Transform B by a Householder matrix Z1: store vector
C describing this temporarily in B and in the local scalar H.
C
CALL DLARFG( N, B(1), B(2), 1, H )
C
B1 = B(1)
B(1) = ONE
C
C Form Z1 * A * Z1.
C
CALL DLARF( 'R', N, N, B, 1, H, A, LDA, DWORK )
CALL DLARF( 'L', N, N, B, 1, H, A, LDA, DWORK )
C
B(1) = B1
TAU(1) = H
ITAU = ITAU + 1
ELSE
B1 = B(1)
END IF
C
C Reduce modified A to upper Hessenberg form by an orthogonal
C similarity transformation with matrix Z2.
C Workspace: need N; prefer N*NB.
C
CALL DGEHRD( N, 1, N, A, LDA, TAU(ITAU), DWORK, LDWORK, INFO )
WRKOPT = DWORK(1)
C
IF ( LJOBZ ) THEN
C
C Save the orthogonal transformations used, so that they could
C be accumulated by calling DORGQR routine.
C
IF ( N.GT.1 )
$ CALL DLACPY( 'F', N-1, 1, B(2), N-1, Z(2,1), LDZ )
IF ( N.GT.2 )
$ CALL DLACPY( 'L', N-2, N-2, A(3,1), LDA, Z(3,2), LDZ )
IF ( LJOBI ) THEN
C
C Form the orthogonal transformation matrix Z = Z1 * Z2.
C Workspace: need N; prefer N*NB.
C
CALL DORGQR( N, N, N, Z, LDZ, TAU, DWORK, LDWORK, INFO )
WRKOPT = MAX( WRKOPT, DWORK(1) )
END IF
END IF
C
C Annihilate the lower part of A and B.
C
IF ( N.GT.2 )
$ CALL DLASET( 'L', N-2, N-2, ZERO, ZERO, A(3,1), LDA )
IF ( N.GT.1 )
$ CALL DLASET( 'F', N-1, 1, ZERO, ZERO, B(2), N-1 )
C
C Find NCONT by checking sizes of the sub-diagonal elements of
C transformed A.
C
IF ( TOL.LE.ZERO ) TOLDEF = THRESH*MAX( FANORM, ABS( B1 ) )
C
J = 1
C
C WHILE ( J < N and ABS( A(J+1,J) ) > TOLDEF ) DO
C
10 CONTINUE
IF ( J.LT.N ) THEN
IF ( ABS( A(J+1,J) ).GT.TOLDEF ) THEN
J = J + 1
GO TO 10
END IF
END IF
C
C END WHILE 10
C
C First negligible sub-diagonal element found, if any: set NCONT.
C
NCONT = J
IF ( J.LT.N ) A(J+1,J) = ZERO
C
C Undo scaling of A and B.
C
CALL MB01PD( 'U', 'H', NCONT, NCONT, 0, 0, ANORM, 0, NBLK, A,
$ LDA, INFO )
CALL MB01PD( 'U', 'G', 1, 1, 0, 0, BNORM, 0, NBLK, B, N, INFO )
IF ( NCONT.LT.N )
$ CALL MB01PD( 'U', 'G', N, N-NCONT, 0, 0, ANORM, 0, NBLK,
$ A(1,NCONT+1), LDA, INFO )
ELSE
C
C B is negligible compared with A. No computations for reducing
C the system to orthogonal canonical form have been performed,
C except scaling (which is undoed).
C
IF( LJOBF ) THEN
CALL DLASET( 'F', N, N, ZERO, ZERO, Z, LDZ )
CALL DLASET( 'F', N, 1, ZERO, ZERO, TAU, N )
ELSE IF( LJOBI ) THEN
CALL DLASET( 'F', N, N, ZERO, ONE, Z, LDZ )
END IF
CALL MB01PD( 'U', 'G', N, N, 0, 0, ANORM, 0, NBLK, A, LDA,
$ INFO )
CALL MB01PD( 'U', 'G', N, 1, 0, 0, BNORM, 0, NBLK, B, N, INFO )
END IF
C
C Set optimal workspace dimension.
C
DWORK(1) = WRKOPT
C
RETURN
C *** Last line of AB01MD ***
END
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