**Purpose**

To estimate the conditioning and compute an error bound on the solution of the real continuous-time matrix algebraic Riccati equation op(A)'*X + X*op(A) + Q - X*G*X = 0, (1) where op(A) = A or A' (A**T) and Q, G are symmetric (Q = Q**T, G = G**T). The matrices A, Q and G are N-by-N and the solution X is N-by-N.

SUBROUTINE SB02QD( JOB, FACT, TRANA, UPLO, LYAPUN, N, A, LDA, T, $ LDT, U, LDU, G, LDG, Q, LDQ, X, LDX, SEP, $ RCOND, FERR, IWORK, DWORK, LDWORK, INFO ) C .. Scalar Arguments .. CHARACTER FACT, JOB, LYAPUN, TRANA, UPLO INTEGER INFO, LDA, LDG, LDQ, LDT, LDU, LDWORK, LDX, N DOUBLE PRECISION FERR, RCOND, SEP C .. Array Arguments .. INTEGER IWORK( * ) DOUBLE PRECISION A( LDA, * ), DWORK( * ), G( LDG, * ), $ Q( LDQ, * ), T( LDT, * ), U( LDU, * ), $ X( LDX, * )

**Mode Parameters**

JOB CHARACTER*1 Specifies the computation to be performed, as follows: = 'C': Compute the reciprocal condition number only; = 'E': Compute the error bound only; = 'B': Compute both the reciprocal condition number and the error bound. FACT CHARACTER*1 Specifies whether or not the real Schur factorization of the matrix Ac = A - G*X (if TRANA = 'N') or Ac = A - X*G (if TRANA = 'T' or 'C') is supplied on entry, as follows: = 'F': On entry, T and U (if LYAPUN = 'O') contain the factors from the real Schur factorization of the matrix Ac; = 'N': The Schur factorization of Ac will be computed and the factors will be stored in T and U (if LYAPUN = 'O'). TRANA CHARACTER*1 Specifies the form of op(A) to be used, as follows: = 'N': op(A) = A (No transpose); = 'T': op(A) = A**T (Transpose); = 'C': op(A) = A**T (Conjugate transpose = Transpose). UPLO CHARACTER*1 Specifies which part of the symmetric matrices Q and G is to be used, as follows: = 'U': Upper triangular part; = 'L': Lower triangular part. LYAPUN CHARACTER*1 Specifies whether or not the original Lyapunov equations should be solved in the iterative estimation process, as follows: = 'O': Solve the original Lyapunov equations, updating the right-hand sides and solutions with the matrix U, e.g., RHS <-- U'*RHS*U; = 'R': Solve reduced Lyapunov equations only, without updating the right-hand sides and solutions.

N (input) INTEGER The order of the matrices A, X, Q, and G. N >= 0. A (input) DOUBLE PRECISION array, dimension (LDA,N) If FACT = 'N' or LYAPUN = 'O', the leading N-by-N part of this array must contain the matrix A. If FACT = 'F' and LYAPUN = 'R', A is not referenced. LDA INTEGER The leading dimension of the array A. LDA >= max(1,N), if FACT = 'N' or LYAPUN = 'O'; LDA >= 1, if FACT = 'F' and LYAPUN = 'R'. T (input or output) DOUBLE PRECISION array, dimension (LDT,N) If FACT = 'F', then T is an input argument and on entry, the leading N-by-N upper Hessenberg part of this array must contain the upper quasi-triangular matrix T in Schur canonical form from a Schur factorization of Ac (see argument FACT). If FACT = 'N', then T is an output argument and on exit, if INFO = 0 or INFO = N+1, the leading N-by-N upper Hessenberg part of this array contains the upper quasi- triangular matrix T in Schur canonical form from a Schur factorization of Ac (see argument FACT). LDT INTEGER The leading dimension of the array T. LDT >= max(1,N). U (input or output) DOUBLE PRECISION array, dimension (LDU,N) If LYAPUN = 'O' and FACT = 'F', then U is an input argument and on entry, the leading N-by-N part of this array must contain the orthogonal matrix U from a real Schur factorization of Ac (see argument FACT). If LYAPUN = 'O' and FACT = 'N', then U is an output argument and on exit, if INFO = 0 or INFO = N+1, it contains the orthogonal N-by-N matrix from a real Schur factorization of Ac (see argument FACT). If LYAPUN = 'R', the array U is not referenced. LDU INTEGER The leading dimension of the array U. LDU >= 1, if LYAPUN = 'R'; LDU >= MAX(1,N), if LYAPUN = 'O'. G (input) DOUBLE PRECISION array, dimension (LDG,N) If UPLO = 'U', the leading N-by-N upper triangular part of this array must contain the upper triangular part of the matrix G. If UPLO = 'L', the leading N-by-N lower triangular part of this array must contain the lower triangular part of the matrix G. _ Matrix G should correspond to G in the "reduced" Riccati equation (with matrix T, instead of A), if LYAPUN = 'R'. See METHOD. LDG INTEGER The leading dimension of the array G. LDG >= max(1,N). Q (input) DOUBLE PRECISION array, dimension (LDQ,N) If UPLO = 'U', the leading N-by-N upper triangular part of this array must contain the upper triangular part of the matrix Q. If UPLO = 'L', the leading N-by-N lower triangular part of this array must contain the lower triangular part of the matrix Q. _ Matrix Q should correspond to Q in the "reduced" Riccati equation (with matrix T, instead of A), if LYAPUN = 'R'. See METHOD. LDQ INTEGER The leading dimension of the array Q. LDQ >= max(1,N). X (input) DOUBLE PRECISION array, dimension (LDX,N) The leading N-by-N part of this array must contain the symmetric solution matrix of the original Riccati equation (with matrix A), if LYAPUN = 'O', or of the "reduced" Riccati equation (with matrix T), if LYAPUN = 'R'. See METHOD. LDX INTEGER The leading dimension of the array X. LDX >= max(1,N). SEP (output) DOUBLE PRECISION If JOB = 'C' or JOB = 'B', the estimated quantity sep(op(Ac),-op(Ac)'). If N = 0, or X = 0, or JOB = 'E', SEP is not referenced. RCOND (output) DOUBLE PRECISION If JOB = 'C' or JOB = 'B', an estimate of the reciprocal condition number of the continuous-time Riccati equation. If N = 0 or X = 0, RCOND is set to 1 or 0, respectively. If JOB = 'E', RCOND is not referenced. FERR (output) DOUBLE PRECISION If JOB = 'E' or JOB = 'B', an estimated forward error bound for the solution X. If XTRUE is the true solution, FERR bounds the magnitude of the largest entry in (X - XTRUE) divided by the magnitude of the largest entry in X. If N = 0 or X = 0, FERR is set to 0. If JOB = 'C', FERR is not referenced.

IWORK INTEGER array, dimension (N*N) DWORK DOUBLE PRECISION array, dimension (LDWORK) On exit, if INFO = 0 or INFO = N+1, DWORK(1) returns the optimal value of LDWORK. LDWORK INTEGER The dimension of the array DWORK. Let LWA = N*N, if LYAPUN = 'O' and JOB = 'E' or 'B'; LWA = 0, otherwise. If FACT = 'N', then LDWORK = MAX(1, 5*N, 2*N*N), if JOB = 'C'; LDWORK = MAX(1, LWA + 5*N, 4*N*N ), if JOB = 'E', 'B'. If FACT = 'F', then LDWORK = MAX(1, 2*N*N), if JOB = 'C'; LDWORK = MAX(1, 4*N*N ), if JOB = 'E' or 'B'. For good performance, LDWORK must generally be larger. If LDWORK = -1, then a workspace query is assumed; the routine only calculates the optimal size of the DWORK array, returns this value as the first entry of the DWORK array, and no error message related to LDWORK is issued by XERBLA.

INFO INTEGER = 0: successful exit; < 0: if INFO = -i, the i-th argument had an illegal value; > 0: if INFO = i, i <= N, the QR algorithm failed to complete the reduction of the matrix Ac to Schur canonical form (see LAPACK Library routine DGEES); on exit, the matrix T(i+1:N,i+1:N) contains the partially converged Schur form, and DWORK(i+1:N) and DWORK(N+i+1:2*N) contain the real and imaginary parts, respectively, of the converged eigenvalues; this error is unlikely to appear; = N+1: if the matrices T and -T' have common or very close eigenvalues; perturbed values were used to solve Lyapunov equations, but the matrix T, if given (for FACT = 'F'), is unchanged.

The condition number of the Riccati equation is estimated as cond = ( norm(Theta)*norm(A) + norm(inv(Omega))*norm(Q) + norm(Pi)*norm(G) ) / norm(X), where Omega, Theta and Pi are linear operators defined by Omega(W) = op(Ac)'*W + W*op(Ac), Theta(W) = inv(Omega(op(W)'*X + X*op(W))), Pi(W) = inv(Omega(X*W*X)), and Ac = A - G*X (if TRANA = 'N') or Ac = A - X*G (if TRANA = 'T' or 'C'). Note that the Riccati equation (1) is equivalent to _ _ _ _ _ _ op(T)'*X + X*op(T) + Q + X*G*X = 0, (2) _ _ _ where X = U'*X*U, Q = U'*Q*U, and G = U'*G*U, with U the orthogonal matrix reducing Ac to a real Schur form, T = U'*Ac*U. The routine estimates the quantities sep(op(Ac),-op(Ac)') = 1 / norm(inv(Omega)), norm(Theta) and norm(Pi) using 1-norm condition estimator. The forward error bound is estimated using a practical error bound similar to the one proposed in [2].

[1] Ghavimi, A.R. and Laub, A.J. Backward error, sensitivity, and refinement of computed solutions of algebraic Riccati equations. Numerical Linear Algebra with Applications, vol. 2, pp. 29-49, 1995. [2] Higham, N.J. Perturbation theory and backward error for AX-XB=C. BIT, vol. 33, pp. 124-136, 1993. [3] Petkov, P.Hr., Konstantinov, M.M., and Mehrmann, V. DGRSVX and DMSRIC: Fortran 77 subroutines for solving continuous-time matrix algebraic Riccati equations with condition and accuracy estimates. Preprint SFB393/98-16, Fak. f. Mathematik, Tech. Univ. Chemnitz, May 1998.

3 The algorithm requires 0(N ) operations. The accuracy of the estimates obtained depends on the solution accuracy and on the properties of the 1-norm estimator.

The option LYAPUN = 'R' may occasionally produce slightly worse or better estimates, and it is much faster than the option 'O'. When SEP is computed and it is zero, the routine returns immediately, with RCOND and FERR (if requested) set to 0 and 1, respectively. In this case, the equation is singular.

**Program Text**

* SB02QD EXAMPLE PROGRAM TEXT * Copyright (c) 2002-2017 NICONET e.V. * * .. Parameters .. INTEGER NIN, NOUT PARAMETER ( NIN = 5, NOUT = 6 ) INTEGER NMAX PARAMETER ( NMAX = 20 ) INTEGER LDA, LDG, LDQ, LDT, LDU, LDX PARAMETER ( LDA = NMAX, LDG = NMAX, LDQ = NMAX, LDT = NMAX, $ LDU = NMAX, LDX = NMAX ) INTEGER LIWORK PARAMETER ( LIWORK = NMAX*NMAX ) INTEGER LDWORK PARAMETER ( LDWORK = 8*NMAX*NMAX + 10*NMAX ) DOUBLE PRECISION ZERO, ONE PARAMETER ( ZERO = 0.0D+0, ONE = 1.0D+0 ) * .. Local Scalars .. DOUBLE PRECISION FERR, RCND, RCOND, SEP INTEGER I, INFO1, INFO2, INFO3, IS, IU, IW, J, N, N2, $ SDIM CHARACTER*1 FACT, JOB, JOBS, LYAPUN, TRANA, TRANAT, UPLO * .. Local Arrays .. LOGICAL BWORK(2*NMAX) INTEGER IWORK(LIWORK) DOUBLE PRECISION A(LDA,NMAX), DWORK(LDWORK), G(LDG,NMAX), $ Q(LDQ,NMAX), T(LDT,NMAX), U(LDU,NMAX), $ X(LDX,NMAX) * .. External Functions .. LOGICAL LSAME, SELECT EXTERNAL LSAME, SELECT * .. External Subroutines .. EXTERNAL DGEES, DLACPY, DSYMM, MA02ED, MB01RU, SB02MD, $ SB02QD * .. Executable Statements .. * WRITE ( NOUT, FMT = 99999 ) * Skip the heading in the data file and read the data. READ ( NIN, FMT = '()' ) READ ( NIN, FMT = * ) N, JOB, FACT, TRANA, UPLO, LYAPUN IF ( N.LT.0 .OR. N.GT.NMAX ) THEN WRITE ( NOUT, FMT = 99993 ) N ELSE READ ( NIN, FMT = * ) ( ( A(I,J), J = 1,N ), I = 1,N ) READ ( NIN, FMT = * ) ( ( Q(I,J), J = 1,N ), I = 1,N ) READ ( NIN, FMT = * ) ( ( G(I,J), J = 1,N ), I = 1,N ) CALL DLACPY( UPLO, N, N, Q, LDQ, X, LDX ) N2 = 2*N IS = 2*N2 + 1 IU = IS + N2*N2 IW = IU + N2*N2 * Solve the continuous-time Riccati equation. CALL SB02MD( 'continuous', 'direct', UPLO, 'no scaling', $ 'stable', N, A, LDA, G, LDG, X, LDX, RCND, $ DWORK(1), DWORK(N2+1), DWORK(IS), N2, DWORK(IU), $ N2, IWORK, DWORK(IW), LDWORK-IW+1, BWORK, INFO1 ) * IF ( INFO1.EQ.0 ) THEN WRITE ( NOUT, FMT = 99995 ) DO 10 I = 1, N WRITE ( NOUT, FMT = 99994 ) ( X(I,J), J = 1,N ) 10 CONTINUE IF ( LSAME( FACT, 'F' ) .OR. LSAME( LYAPUN, 'R' ) ) THEN CALL DLACPY( 'Full', N, N, A, LDA, T, LDT ) IF ( LSAME( TRANA, 'N' ) ) THEN * Compute Ac = A-G*X. CALL DSYMM( 'Left', UPLO, N, N, -ONE, G, LDG, X, LDX, $ ONE, T, LDT ) ELSE * Compute Ac = A-X*G. CALL DSYMM( 'Right', UPLO, N, N, -ONE, G, LDG, X, LDX, $ ONE, T, LDT ) END IF * Compute the Schur factorization of Ac. JOBS = 'V' CALL DGEES( JOBS, 'Not ordered', SELECT, N, T, LDT, SDIM, $ DWORK(1), DWORK(N+1), U, LDU, DWORK(2*N+1), $ LDWORK-2*N, BWORK, INFO3 ) IF( INFO3.NE.0 ) THEN WRITE ( NOUT, FMT = 99996 ) INFO3 STOP END IF END IF * IF ( LSAME( LYAPUN, 'R' ) ) THEN IF( LSAME( TRANA, 'N' ) ) THEN TRANAT = 'T' ELSE TRANAT = 'N' END IF * CALL MB01RU( UPLO, TRANAT, N, N, ZERO, ONE, X, LDX, $ U, LDU, X, LDX, DWORK, N*N, INFO2 ) CALL MA02ED( UPLO, N, X, LDX ) CALL MB01RU( UPLO, TRANAT, N, N, ZERO, ONE, G, LDG, $ U, LDU, G, LDG, DWORK, N*N, INFO2 ) CALL MB01RU( UPLO, TRANAT, N, N, ZERO, ONE, Q, LDQ, $ U, LDU, Q, LDQ, DWORK, N*N, INFO2 ) END IF * Estimate the condition and error bound on the solution. CALL SB02QD( JOB, FACT, TRANA, UPLO, LYAPUN, N, A, LDA, T, $ LDT, U, LDU, G, LDG, Q, LDQ, X, LDX, SEP, $ RCOND, FERR, IWORK, DWORK, LDWORK, INFO2 ) * IF ( INFO2.NE.0 ) THEN WRITE ( NOUT, FMT = 99997 ) INFO2 END IF IF ( INFO2.EQ.0 .OR. INFO2.EQ.N+1 ) THEN WRITE ( NOUT, FMT = 99992 ) SEP WRITE ( NOUT, FMT = 99991 ) RCOND WRITE ( NOUT, FMT = 99990 ) FERR END IF ELSE WRITE ( NOUT, FMT = 99998 ) INFO1 END IF END IF STOP * 99999 FORMAT (' SB02QD EXAMPLE PROGRAM RESULTS',/1X) 99998 FORMAT (' INFO on exit from SB02MD =',I2) 99997 FORMAT (' INFO on exit from SB02QD =',I2) 99996 FORMAT (' INFO on exit from DGEES =',I2) 99995 FORMAT (' The solution matrix X is') 99994 FORMAT (20(1X,F8.4)) 99993 FORMAT (/' N is out of range.',/' N = ',I5) 99992 FORMAT (/' Estimated separation = ',F8.4) 99991 FORMAT (/' Estimated reciprocal condition number = ',F8.4) 99990 FORMAT (/' Estimated error bound = ',F8.4) END

SB02QD EXAMPLE PROGRAM DATA 2 B N N U O 0.0 1.0 0.0 0.0 1.0 0.0 0.0 2.0 0.0 0.0 0.0 1.0

SB02QD EXAMPLE PROGRAM RESULTS The solution matrix X is 2.0000 1.0000 1.0000 2.0000 Estimated separation = 0.4000 Estimated reciprocal condition number = 0.1333 Estimated error bound = 0.0000