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SUBROUTINE MB02WD( FORM, F, N, IPAR, LIPAR, DPAR, LDPAR, ITMAX,
$ A, LDA, B, INCB, X, INCX, TOL, DWORK, LDWORK,
$ IWARN, 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 solve the system of linear equations Ax = b, with A symmetric,
C positive definite, or, in the implicit form, f(A, x) = b, where
C y = f(A, x) is a symmetric positive definite linear mapping
C from x to y, using the conjugate gradient (CG) algorithm without
C preconditioning.
C
C ARGUMENTS
C
C Mode Parameters
C
C FORM CHARACTER*1
C Specifies the form of the system of equations, as
C follows:
C = 'U' : Ax = b, the upper triagular part of A is used;
C = 'L' : Ax = b, the lower triagular part of A is used;
C = 'F' : the implicit, function form, f(A, x) = b.
C
C Function Parameters
C
C F EXTERNAL
C If FORM = 'F', then F is a subroutine which calculates the
C value of f(A, x), for given A and x.
C If FORM <> 'F', then F is not called.
C
C F must have the following interface:
C
C SUBROUTINE F( N, IPAR, LIPAR, DPAR, LDPAR, A, LDA, X,
C $ INCX, DWORK, LDWORK, INFO )
C
C where
C
C N (input) INTEGER
C The dimension of the vector x. N >= 0.
C
C IPAR (input) INTEGER array, dimension (LIPAR)
C The integer parameters describing the structure of
C the matrix A.
C
C LIPAR (input) INTEGER
C The length of the array IPAR. LIPAR >= 0.
C
C DPAR (input) DOUBLE PRECISION array, dimension (LDPAR)
C The real parameters needed for solving the
C problem.
C
C LDPAR (input) INTEGER
C The length of the array DPAR. LDPAR >= 0.
C
C A (input) DOUBLE PRECISION array, dimension
C (LDA, NC), where NC is the number of columns.
C The leading NR-by-NC part of this array must
C contain the (compressed) representation of the
C matrix A, where NR is the number of rows of A
C (function of IPAR entries).
C
C LDA (input) INTEGER
C The leading dimension of the array A.
C LDA >= MAX(1,NR).
C
C X (input/output) DOUBLE PRECISION array, dimension
C (1+(N-1)*INCX)
C On entry, this incremented array must contain the
C vector x.
C On exit, this incremented array contains the value
C of the function f, y = f(A, x).
C
C INCX (input) INTEGER
C The increment for the elements of X. INCX > 0.
C
C DWORK DOUBLE PRECISION array, dimension (LDWORK)
C The workspace array for subroutine F.
C
C LDWORK (input) INTEGER
C The size of the array DWORK (as large as needed
C in the subroutine F).
C
C INFO INTEGER
C Error indicator, set to a negative value if an
C input scalar argument is erroneous, and to
C positive values for other possible errors in the
C subroutine F. The LAPACK Library routine XERBLA
C should be used in conjunction with negative INFO.
C INFO must be zero if the subroutine finished
C successfully.
C
C Parameters marked with "(input)" must not be changed.
C
C Input/Output Parameters
C
C N (input) INTEGER
C The dimension of the vector x. N >= 0.
C If FORM = 'U' or FORM = 'L', N is also the number of rows
C and columns of the matrix A.
C
C IPAR (input) INTEGER array, dimension (LIPAR)
C If FORM = 'F', the integer parameters describing the
C structure of the matrix A.
C This parameter is ignored if FORM = 'U' or FORM = 'L'.
C
C LIPAR (input) INTEGER
C The length of the array IPAR. LIPAR >= 0.
C
C DPAR (input) DOUBLE PRECISION array, dimension (LDPAR)
C If FORM = 'F', the real parameters needed for solving
C the problem.
C This parameter is ignored if FORM = 'U' or FORM = 'L'.
C
C LDPAR (input) INTEGER
C The length of the array DPAR. LDPAR >= 0.
C
C ITMAX (input) INTEGER
C The maximal number of iterations to do. ITMAX >= 0.
C
C A (input) DOUBLE PRECISION array,
C dimension (LDA, NC), if FORM = 'F',
C dimension (LDA, N), otherwise.
C If FORM = 'F', the leading NR-by-NC part of this array
C must contain the (compressed) representation of the
C matrix A, where NR and NC are the number of rows and
C columns, respectively, of the matrix A. The array A is
C not referenced by this routine itself, except in the
C calls to the routine F.
C If FORM <> 'F', the leading N-by-N part of this array
C must contain the matrix A, assumed to be symmetric;
C only the triangular part specified by FORM is referenced.
C
C LDA (input) INTEGER
C The leading dimension of array A.
C LDA >= MAX(1,NR), if FORM = 'F';
C LDA >= MAX(1,N), if FORM = 'U' or FORM = 'L'.
C
C B (input) DOUBLE PRECISION array, dimension (1+(N-1)*INCB)
C The incremented vector b.
C
C INCB (input) INTEGER
C The increment for the elements of B. INCB > 0.
C
C X (input/output) DOUBLE PRECISION array, dimension
C (1+(N-1)*INCX)
C On entry, this incremented array must contain an initial
C approximation of the solution. If an approximation is not
C known, setting all elements of x to zero is recommended.
C On exit, this incremented array contains the computed
C solution x of the system of linear equations.
C
C INCX (input) INTEGER
C The increment for the elements of X. INCX > 0.
C
C Tolerances
C
C TOL DOUBLE PRECISION
C If TOL > 0, absolute tolerance for the iterative process.
C The algorithm will stop if || Ax - b ||_2 <= TOL. Since
C it is advisable to use a relative tolerance, say TOLER,
C TOL should be chosen as TOLER*|| b ||_2.
C If TOL <= 0, a default relative tolerance,
C TOLDEF = N*EPS*|| b ||_2, is used, where EPS is the
C machine precision (see LAPACK Library routine DLAMCH).
C
C Workspace
C
C DWORK DOUBLE PRECISION array, dimension (LDWORK)
C On exit, if INFO = 0, DWORK(1) returns the number of
C iterations performed and DWORK(2) returns the remaining
C residual, || Ax - b ||_2.
C
C LDWORK INTEGER
C The length of the array DWORK.
C LDWORK >= MAX(2,3*N + DWORK(F)), if FORM = 'F',
C where DWORK(F) is the workspace needed by F;
C LDWORK >= MAX(2,3*N), if FORM = 'U' or FORM = 'L'.
C
C Warning Indicator
C
C IWARN INTEGER
C = 0: no warning;
C = 1: the algorithm finished after ITMAX > 0 iterations,
C without achieving the desired precision TOL;
C = 2: ITMAX is zero; in this case, DWORK(2) is not set.
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 > 0: if INFO = i, then F returned with INFO = i.
C
C METHOD
C
C The following CG iteration is used for solving Ax = b:
C
C Start: q(0) = r(0) = Ax - b
C
C < q(k), r(k) >
C ALPHA(k) = - ----------------
C < q(k), Aq(k) >
C x(k+1) = x(k) - ALPHA(k) * q(k)
C r(k+1) = r(k) - ALPHA(k) * Aq(k)
C < r(k+1), r(k+1) >
C BETA(k) = --------------------
C < r(k) , r(k) >
C q(k+1) = r(k+1) + BETA(k) * q(k)
C
C where <.,.> denotes the scalar product.
C
C REFERENCES
C
C [1] Golub, G.H. and van Loan, C.F.
C Matrix Computations. Third Edition.
C M. D. Johns Hopkins University Press, Baltimore, pp. 520-528,
C 1996.
C
C [2] Luenberger, G.
C Introduction to Linear and Nonlinear Programming.
C Addison-Wesley, Reading, MA, p.187, York, 1973.
C
C NUMERICAL ASPECTS
C
C Since the residuals are orthogonal in the scalar product
C <x, y> = y'Ax, the algorithm is theoretically finite. But rounding
C errors cause a loss of orthogonality, so a finite termination
C cannot be guaranteed. However, one can prove [2] that
C
C || x-x_k ||_A := sqrt( (x-x_k)' * A * (x-x_k) )
C
C sqrt( kappa_2(A) ) - 1
C <= 2 || x-x_0 ||_A * ------------------------ ,
C sqrt( kappa_2(A) ) + 1
C
C where kappa_2 is the condition number.
C
C The approximate number of floating point operations is
C (k*(N**2 + 15*N) + N**2 + 3*N)/2, if FORM <> 'F',
C k*(f + 7*N) + f, if FORM = 'F',
C where k is the number of CG iterations performed, and f is the
C number of floating point operations required by the subroutine F.
C
C CONTRIBUTORS
C
C A. Riedel, R. Schneider, Chemnitz University of Technology,
C Oct. 2000, during a stay at University of Twente, NL.
C
C REVISIONS
C
C V. Sima, Research Institute for Informatics, Bucharest, Mar. 2001,
C March, 2002.
C
C KEYWORDS
C
C Conjugate gradients, convergence, linear system of equations,
C matrix operations.
C
C ******************************************************************
C
C .. Parameters ..
DOUBLE PRECISION ZERO, ONE
PARAMETER ( ZERO = 0.0D0, ONE = 1.0D0 )
C .. Scalar Arguments ..
CHARACTER FORM
INTEGER INCB, INCX, INFO, ITMAX, IWARN, LDA, LDPAR,
$ LDWORK, LIPAR, N
DOUBLE PRECISION TOL
C .. Array Arguments ..
DOUBLE PRECISION A(LDA,*), B(*), DPAR(*), DWORK(*), X(*)
INTEGER IPAR(*)
C .. Local Scalars ..
DOUBLE PRECISION ALPHA, BETA, RES, RESOLD, TOLDEF
INTEGER AQ, DWLEFT, K, R
LOGICAL MAT
C .. External Functions ..
DOUBLE PRECISION DDOT, DLAMCH, DNRM2
LOGICAL LSAME
EXTERNAL DDOT, DLAMCH, DNRM2, LSAME
C .. External Subroutines ..
EXTERNAL DAXPY, DCOPY, DSCAL, DSYMV, F, XERBLA
C .. Intrinsic Functions ..
INTRINSIC MAX
C ..
C .. Executable Statements ..
C
C Decode the scalar input parameters.
C
MAT = LSAME( FORM, 'U' ) .OR. LSAME( FORM, 'L' )
C
C Check the scalar input parameters.
C
IWARN = 0
INFO = 0
IF( .NOT.( MAT .OR. LSAME( FORM, 'F' ) ) ) THEN
INFO = -1
ELSEIF ( N.LT.0 ) THEN
INFO = -3
ELSEIF ( .NOT. MAT .AND. LIPAR.LT.0 ) THEN
INFO = -5
ELSEIF ( .NOT. MAT .AND. LDPAR.LT.0 ) THEN
INFO = -7
ELSEIF ( ITMAX.LT.0 ) THEN
INFO = -8
ELSEIF ( LDA.LT.1 .OR. ( MAT .AND. LDA.LT.N ) ) THEN
INFO = -10
ELSEIF ( INCB.LE.0 ) THEN
INFO = -12
ELSEIF ( INCX.LE.0 ) THEN
INFO = -14
ELSEIF ( LDWORK.LT.MAX( 2, 3*N ) ) THEN
INFO = -17
ENDIF
C
C Return if there are illegal arguments.
C
IF( INFO.NE.0 ) THEN
CALL XERBLA( 'MB02WD', -INFO )
RETURN
ENDIF
C
C Quick return if possible.
C
IF ( N.EQ.0 ) THEN
DWORK(1) = ZERO
DWORK(2) = ZERO
RETURN
ENDIF
C
IF ( ITMAX.EQ.0 ) THEN
DWORK(1) = ZERO
IWARN = 2
RETURN
ENDIF
C
C Set default tolerance, if needed.
C
TOLDEF = TOL
IF ( TOLDEF.LE.ZERO )
$ TOLDEF = DBLE( N )*DLAMCH( 'Epsilon' )*DNRM2( N, B, INCB )
C
C Initialize local variables.
C
K = 0
C
C Vector q is stored in DWORK(1), A*q or f(A, q) in DWORK(AQ),
C and r in DWORK(R). The workspace for F starts in DWORK(DWLEFT).
C
AQ = N + 1
R = N + AQ
DWLEFT = N + R
C
C Prepare the first iteration, initialize r and q.
C
IF ( MAT ) THEN
CALL DCOPY( N, B, INCB, DWORK(R), 1 )
CALL DSYMV( FORM, N, ONE, A, LDA, X, INCX, -ONE, DWORK(R), 1 )
ELSE
CALL DCOPY( N, X, INCX, DWORK(R), 1 )
CALL F( N, IPAR, LIPAR, DPAR, LDPAR, A, LDA, DWORK(R), 1,
$ DWORK(DWLEFT), LDWORK-DWLEFT+1, INFO )
IF ( INFO.NE.0 )
$ RETURN
CALL DAXPY( N, -ONE, B, INCB, DWORK(R), 1 )
ENDIF
CALL DCOPY( N, DWORK(R), 1, DWORK, 1 )
C
RES = DNRM2( N, DWORK(R), 1 )
C
C Do nothing if x is already the solution.
C
IF ( RES.LE.TOLDEF ) GOTO 20
C
C Begin of the iteration loop.
C
C WHILE ( RES.GT.TOLDEF .AND. K.LE.ITMAX ) DO
10 CONTINUE
C
C Calculate A*q or f(A, q).
C
IF ( MAT ) THEN
CALL DSYMV( FORM, N, ONE, A, LDA, DWORK, 1, ZERO, DWORK(AQ),
$ 1 )
ELSE
CALL DCOPY( N, DWORK, 1, DWORK(AQ), 1 )
CALL F( N, IPAR, LIPAR, DPAR, LDPAR, A, LDA, DWORK(AQ), 1,
$ DWORK(DWLEFT), LDWORK-DWLEFT+1, INFO )
IF ( INFO.NE.0 )
$ RETURN
ENDIF
C
C Calculate ALPHA(k).
C
ALPHA = DDOT( N, DWORK, 1, DWORK(R), 1 ) /
$ DDOT( N, DWORK, 1, DWORK(AQ), 1 )
C
C x(k+1) = x(k) - ALPHA(k)*q(k).
C
CALL DAXPY( N, -ALPHA, DWORK, 1, X, INCX )
C
C r(k+1) = r(k) - ALPHA(k)*(A*q(k)).
C
CALL DAXPY( N, -ALPHA, DWORK(AQ), 1, DWORK(R), 1 )
C
C Save RES and calculate a new RES.
C
RESOLD = RES
RES = DNRM2( N, DWORK(R), 1 )
C
C Exit if tolerance is reached.
C
IF ( RES.LE.TOLDEF ) GOTO 20
C
C Calculate BETA(k).
C
BETA = ( RES/RESOLD )**2
C
C q(k+1) = r(k+1) + BETA(k)*q(k).
C
CALL DSCAL( N, BETA, DWORK, 1 )
CALL DAXPY( N, ONE, DWORK(R), 1, DWORK, 1 )
C
C End of the iteration loop.
C
K = K + 1
IF ( K.LT.ITMAX ) GOTO 10
C END WHILE 10
C
C Tolerance was not reached!
C
IWARN = 1
C
20 CONTINUE
C
DWORK(1) = K
DWORK(2) = RES
C
C *** Last line of MB02WD ***
END
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