## SB02QD

### Estimating conditioning and forward error bound for the solution of continuous-time Riccati equation

[Specification] [Arguments] [Method] [References] [Comments] [Example]

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.

```
Specification
```      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, * )

```
Arguments

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.

```
Input/Output Parameters
```  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.

```
Workspace
```  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.

```
Error Indicator
```  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.

```
Method
```  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 .

```
References
```   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.

 Higham, N.J.
Perturbation theory and backward error for AX-XB=C.
BIT, vol. 33, pp. 124-136, 1993.

 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.

```
Numerical Aspects
```                            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.

```
Example

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
```
Program Data
``` 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
```
Program Results
``` 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
```