**Purpose**

To compute the optimal feedback matrix F for the problem of optimal control given by -1 F = (R + B'XB) (B'XA + L') (1) in the discrete-time case and -1 F = R (B'X + L') (2) in the continuous-time case, where A, B and L are N-by-N, N-by-M and N-by-M matrices respectively; R and X are M-by-M and N-by-N symmetric matrices respectively. Optionally, matrix R may be specified in a factored form, and L may be zero.

SUBROUTINE SB02ND( DICO, FACT, UPLO, JOBL, N, M, P, A, LDA, B, $ LDB, R, LDR, IPIV, L, LDL, X, LDX, RNORM, F, $ LDF, OUFACT, IWORK, DWORK, LDWORK, INFO ) C .. Scalar Arguments .. CHARACTER DICO, FACT, JOBL, UPLO INTEGER INFO, LDA, LDB, LDF, LDL, LDR, LDWORK, LDX, M, $ N, P DOUBLE PRECISION RNORM C .. Array Arguments .. INTEGER IPIV(*), IWORK(*), OUFACT(2) DOUBLE PRECISION A(LDA,*), B(LDB,*), DWORK(*), F(LDF,*), $ L(LDL,*), R(LDR,*), X(LDX,*)

**Mode Parameters**

DICO CHARACTER*1 Specifies the equation from which F is to be determined, as follows: = 'D': Equation (1), discrete-time case; = 'C': Equation (2), continuous-time case. FACT CHARACTER*1 Specifies how the matrix R is given (factored or not), as follows: = 'N': Array R contains the matrix R; = 'D': Array R contains a P-by-M matrix D, where R = D'D; = 'C': Array R contains the Cholesky factor of R; = 'U': Array R contains the symmetric indefinite UdU' or LdL' factorization of R. This option is not available for DICO = 'D'. UPLO CHARACTER*1 Specifies which triangle of the possibly factored matrix R (or R + B'XB, on exit) is or should be stored, as follows: = 'U': Upper triangle is stored; = 'L': Lower triangle is stored. JOBL CHARACTER*1 Specifies whether or not the matrix L is zero, as follows: = 'Z': L is zero; = 'N': L is nonzero.

N (input) INTEGER The order of the matrices A and X. N >= 0. No computations are performed if MIN(N,M) = 0. M (input) INTEGER The number of system inputs. M >= 0. P (input) INTEGER The number of rows of the matrix D. P >= M for DICO = 'C'; P >= 0 for DICO = 'D'. This parameter must be specified only for FACT = 'D'. A (input) DOUBLE PRECISION array, dimension (LDA,N) If DICO = 'D', the leading N-by-N part of this array must contain the state matrix A of the system. If DICO = 'C', this array is not referenced. LDA INTEGER The leading dimension of array A. LDA >= MAX(1,N) if DICO = 'D'; LDA >= 1 if DICO = 'C'. B (input/worksp.) DOUBLE PRECISION array, dimension (LDB,M) The leading N-by-M part of this array must contain the input matrix B of the system. If DICO = 'D' and FACT = 'D' or 'C', the contents of this array is destroyed. Specifically, if, on exit, OUFACT(2) = 1, this array contains chol(X)*B, and if OUFACT(2) = 2 and INFO < M+2, but INFO >= 0, its trailing part (in the first N rows) contains the submatrix of sqrt(V)*U'B corresponding to the non-negligible, positive eigenvalues of X, where V and U are the matrices with the eigenvalues and eigenvectors of X. Otherwise, B is unchanged on exit. LDB INTEGER The leading dimension of array B. LDB >= MAX(1,N). R (input/output) DOUBLE PRECISION array, dimension (LDR,M) On entry, if FACT = 'N', the leading M-by-M upper triangular part (if UPLO = 'U') or lower triangular part (if UPLO = 'L') of this array must contain the upper triangular part or lower triangular part, respectively, of the symmetric input weighting matrix R. On entry, if FACT = 'D', the leading P-by-M part of this array must contain the direct transmission matrix D of the system. On entry, if FACT = 'C', the leading M-by-M upper triangular part (if UPLO = 'U') or lower triangular part (if UPLO = 'L') of this array must contain the Cholesky factor of the positive definite input weighting matrix R (as produced by LAPACK routine DPOTRF). On entry, if DICO = 'C' and FACT = 'U', the leading M-by-M upper triangular part (if UPLO = 'U') or lower triangular part (if UPLO = 'L') of this array must contain the factors of the UdU' or LdL' factorization, respectively, of the symmetric indefinite input weighting matrix R (as produced by LAPACK routine DSYTRF). The strictly lower triangular part (if UPLO = 'U') or strictly upper triangular part (if UPLO = 'L') of this array is used as workspace (filled in by symmetry with the other strictly triangular part of R, of R+B'XB, or of the result, if DICO = 'C', DICO = 'D', or (DICO = 'D' and (FACT = 'D' or FACT = 'C') and UPLO = 'L'), respectively. On exit, if OUFACT(1) = 1, and INFO = 0 (or INFO = M+1), the leading M-by-M upper triangular part (if UPLO = 'U') or lower triangular part (if UPLO = 'L') of this array contains the Cholesky factor of the given input weighting matrix R (for DICO = 'C'), or that of the matrix R + B'XB (for DICO = 'D'). On exit, if OUFACT(1) = 2, and INFO = 0 (or INFO = M+1), the leading M-by-M upper triangular part (if UPLO = 'U') or lower triangular part (if UPLO = 'L') of this array contains the factors of the UdU' or LdL' factorization, respectively, of the given input weighting matrix (for DICO = 'C'), or that of the matrix R + B'XB (for DICO = 'D' and FACT = 'N'). On exit R is unchanged if FACT = 'U' or N = 0. LDR INTEGER. The leading dimension of the array R. LDR >= MAX(1,M) if FACT <> 'D'; LDR >= MAX(1,M,P) if FACT = 'D'. IPIV (input/output) INTEGER array, dimension (M) On entry, if FACT = 'U', this array must contain details of the interchanges performed and the block structure of the d factor in the UdU' or LdL' factorization of matrix R (as produced by LAPACK routine DSYTRF). On exit, if OUFACT(1) = 2, this array contains details of the interchanges performed and the block structure of the d factor in the UdU' or LdL' factorization of matrix R or R + B'XB, as produced by LAPACK routine DSYTRF. This array is not referenced if FACT = 'D', or FACT = 'C', or N = 0. L (input) DOUBLE PRECISION array, dimension (LDL,M) If JOBL = 'N', the leading N-by-M part of this array must contain the cross weighting matrix L. If JOBL = 'Z', this array is not referenced. LDL INTEGER The leading dimension of array L. LDL >= MAX(1,N) if JOBL = 'N'; LDL >= 1 if JOBL = 'Z'. X (input/output) DOUBLE PRECISION array, dimension (LDX,N) On entry, the leading N-by-N part of this array must contain the solution matrix X of the algebraic Riccati equation as produced by SLICOT Library routines SB02MD or SB02OD. Matrix X is assumed non-negative definite if DICO = 'D' and (FACT = 'D' or FACT = 'C'). The full matrix X must be given on input if LDWORK < N*M or if DICO = 'D' and (FACT = 'D' or FACT = 'C'). On exit, if DICO = 'D', FACT = 'D' or FACT = 'C', and OUFACT(2) = 1, the N-by-N upper triangular part (if UPLO = 'U') or lower triangular part (if UPLO = 'L') of this array contains the Cholesky factor of the given matrix X, which is found to be positive definite. On exit, if DICO = 'D', FACT = 'D' or 'C', OUFACT(2) = 2, and INFO < M+2 (but INFO >= 0), the leading N-by-N part of this array contains the matrix of orthonormal eigenvectors of X. On exit X is unchanged if DICO = 'C' or FACT = 'N'. LDX INTEGER The leading dimension of array X. LDX >= MAX(1,N). RNORM (input) DOUBLE PRECISION If FACT = 'U', this parameter must contain the 1-norm of the original matrix R (before factoring it). Otherwise, this parameter is not used. F (output) DOUBLE PRECISION array, dimension (LDF,N) The leading M-by-N part of this array contains the optimal feedback matrix F. This array is not referenced if DICO = 'C' and FACT = 'D' and P < M. LDF INTEGER The leading dimension of array F. LDF >= MAX(1,M). OUFACT (output) INTEGER array, dimension (2) Information about the factorization finally used. OUFACT(1) = 1: Cholesky factorization of R (or R + B'XB) has been used; OUFACT(1) = 2: UdU' (if UPLO = 'U') or LdL' (if UPLO = 'L') factorization of R (or R + B'XB) has been used; OUFACT(2) = 1: Cholesky factorization of X has been used; OUFACT(2) = 2: Spectral factorization of X has been used. The value of OUFACT(2) is not set for DICO = 'C' or for DICO = 'D' and FACT = 'N'. This array is not set if N = 0 or M = 0.

IWORK INTEGER array, dimension (M) DWORK DOUBLE PRECISION array, dimension (LDWORK) On exit, if INFO = 0 or LDWORK = -1, DWORK(1) returns the optimal value of LDWORK, and for LDWORK set as specified below, DWORK(2) contains the reciprocal condition number of the matrix R (for DICO = 'C') or of R + B'XB (for DICO = 'D'); DWORK(2) is set to 1 if N = 0. On exit, if LDWORK = -2 on input or INFO = -25, then DWORK(1) returns the minimal value of LDWORK. If on exit INFO = 0, and OUFACT(2) = 2, then DWORK(3),..., DWORK(N+2) contain the eigenvalues of X, in ascending order. LDWORK INTEGER Dimension of working array DWORK. LDWORK >= max(2,2*M) if FACT = 'U'; LDWORK >= max(2,3*M) if FACT <> 'U', DICO = 'C'; LDWORK >= max(2,3*M,N) if FACT = 'N', DICO = 'D'; LDWORK >= max(N+3*M+2,4*N+1) if FACT <> 'N', DICO = 'D'. For optimum performance LDWORK should be larger. If LDWORK = -1, an optimal 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 is issued by XERBLA. If LDWORK = -2, a minimal workspace query is assumed; the routine only calculates the minimal size of the DWORK array, returns this value as the first entry of the DWORK array, and no error message is issued by XERBLA.

INFO INTEGER = 0: successful exit; < 0: if INFO = -i, the i-th argument had an illegal value; = i: if the i-th element of the d factor is exactly zero; the UdU' (or LdL') factorization has been completed, but the block diagonal matrix d is exactly singular; = M+1: if the matrix R (if DICO = 'C'), or R + B'XB (if DICO = 'D') is numerically singular (to working precision); = M+2: if one or more of the eigenvalues of X has not converged; = M+3: if the matrix X is indefinite and updating the triangular factorization failed. If INFO > M+1, call the routine again with an appropriate, unfactored matrix R.

The optimal feedback matrix F is obtained as the solution to the system of linear equations (R + B'XB) * F = B'XA + L' in the discrete-time case and R * F = B'X + L' in the continuous-time case, with R replaced by D'D if FACT = 'D'. If FACT = 'N', Cholesky factorization is tried first, but if the coefficient matrix is not positive definite, then UdU' (or LdL') factorization is used. If FACT <> 'N', the factored form of R is taken into account. The discrete-time case then involves updating of a triangular factorization of R (or D'D); Cholesky or symmetric spectral factorization of X is employed to avoid squaring of the condition number of the matrix. When D is given, its QR factorization is determined, and the triangular factor is used as described above.

The algorithm consists of numerically stable steps. 3 2 For DICO = 'C', it requires O(m + mn ) floating point operations 2 if FACT = 'N' and O(mn ) floating point operations, otherwise. For DICO = 'D', the operation counts are similar, but additional 3 O(n ) floating point operations may be needed in the worst case. These estimates assume that M <= N.

None

**Program Text**

* SB02ND EXAMPLE PROGRAM TEXT * Copyright (c) 2002-2017 NICONET e.V. * * .. Parameters .. INTEGER NIN, NOUT PARAMETER ( NIN = 5, NOUT = 6 ) INTEGER NMAX, MMAX, PMAX PARAMETER ( NMAX = 20, MMAX = 20, PMAX = 20 ) INTEGER NMAX2 PARAMETER ( NMAX2 = 2*NMAX ) INTEGER LDA, LDB, LDC, LDL, LDR, LDS, LDT, LDU, LDX, LDF PARAMETER ( LDA = NMAX, LDB = NMAX, LDC = PMAX, LDL = NMAX, $ LDR = MAX(MMAX,PMAX), LDS = NMAX2+MMAX, $ LDT = NMAX2+MMAX, LDU = NMAX2, LDX = NMAX, $ LDF = MMAX ) INTEGER LIWORK PARAMETER ( LIWORK = MAX( NMAX2,MMAX ) ) INTEGER LDWORK PARAMETER ( LDWORK = MAX( NMAX+3*MMAX+2, 14*NMAX+23, $ 16*NMAX ) ) * .. Local Scalars .. DOUBLE PRECISION TOL, RCOND, RNORM INTEGER I, INFO1, INFO2, J, M, N, P CHARACTER*1 DICO, FACT, JOBB, JOBL, SORT, UPLO * .. Local Arrays .. DOUBLE PRECISION A(LDA,NMAX), ALFAI(2*NMAX), ALFAR(2*NMAX), $ B(LDB,MMAX), BETA(2*NMAX), C(LDC,NMAX), $ DWORK(LDWORK), F(LDF,NMAX), L(LDL,MMAX), $ R(LDR,MMAX), S(LDS,NMAX2+MMAX), T(LDT,NMAX2), $ U(LDU,NMAX2), X(LDX,NMAX) INTEGER IPIV(LIWORK), IWORK(LIWORK), OUFACT(2) LOGICAL BWORK(NMAX2) * .. External Functions .. LOGICAL LSAME EXTERNAL LSAME * .. External Subroutines .. EXTERNAL SB02ND, SB02OD * .. Intrinsic Functions .. INTRINSIC MAX * .. Executable Statements .. * WRITE ( NOUT, FMT = 99999 ) * Skip the heading in the data file and read the data. READ ( NIN, FMT = '()' ) READ ( NIN, FMT = * ) N, M, P, TOL, DICO, FACT, JOBL, UPLO 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 ) IF ( M.LT.0 .OR. M.GT.MMAX ) THEN WRITE ( NOUT, FMT = 99992 ) M ELSE READ ( NIN, FMT = * ) ( ( B(I,J), J = 1,M ), I = 1,N ) IF ( P.LT.0 .OR. P.GT.PMAX ) THEN WRITE ( NOUT, FMT = 99991 ) P ELSE READ ( NIN, FMT = * ) ( ( C(I,J), J = 1,N ), I = 1,P ) IF ( LSAME( FACT, 'D' ) ) THEN READ ( NIN, FMT = * ) ( ( R(I,J), J = 1,M ), I = 1,P ) ELSE READ ( NIN, FMT = * ) ( ( R(I,J), J = 1,M ), I = 1,M ) END IF * Find the solution matrix X. JOBB = 'B' SORT = 'S' CALL SB02OD( DICO, JOBB, 'Both', UPLO, JOBL, SORT, N, M, $ P, A, LDA, B, LDB, C, LDC, R, LDR, L, LDL, $ RCOND, X, LDX, ALFAR, ALFAI, BETA, S, LDS, $ T, LDT, U, LDU, TOL, IWORK, DWORK, LDWORK, $ BWORK, INFO1 ) * IF ( INFO1.NE.0 ) THEN WRITE ( NOUT, FMT = 99998 ) INFO1 ELSE WRITE ( NOUT, FMT = 99996 ) DO 20 I = 1, N WRITE ( NOUT, FMT = 99994 ) ( X(I,J), J = 1,N ) 20 CONTINUE * Compute the optimal feedback matrix F. CALL SB02ND( DICO, FACT, UPLO, JOBL, N, M, P, A, LDA, $ B, LDB, R, LDR, IPIV, L, LDL, X, LDX, $ RNORM, F, LDF, OUFACT, IWORK, DWORK, $ LDWORK, INFO2 ) * IF ( INFO2.NE.0 ) THEN WRITE ( NOUT, FMT = 99997 ) INFO2 ELSE WRITE ( NOUT, FMT = 99995 ) DO 40 I = 1, M WRITE ( NOUT, FMT = 99994 ) ( F(I,J), J = 1,N ) 40 CONTINUE END IF END IF END IF END IF END IF STOP * 99999 FORMAT (' SB02ND EXAMPLE PROGRAM RESULTS',/1X) 99998 FORMAT (' INFO on exit from SB02OD = ',I2) 99997 FORMAT (' INFO on exit from SB02ND = ',I2) 99996 FORMAT (' The solution matrix X is ') 99995 FORMAT (/' The optimal feedback matrix F is ') 99994 FORMAT (20(1X,F8.4)) 99993 FORMAT (/' N is out of range.',/' N = ',I5) 99992 FORMAT (/' M is out of range.',/' M = ',I5) 99991 FORMAT (/' P is out of range.',/' P = ',I5) END

SB02ND EXAMPLE PROGRAM DATA 2 1 3 0.0 D N Z U 2.0 -1.0 1.0 0.0 1.0 0.0 0.0 0.0 0.0 0.0 0.0 1.0 0.0 0.0 0.0

SB02ND EXAMPLE PROGRAM RESULTS The solution matrix X is 1.0000 0.0000 0.0000 1.0000 The optimal feedback matrix F is 2.0000 -1.0000