## SB03TD

### Solution of continuous-time Lyapunov equations and condition and error bounds estimation

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

Purpose

      To solve the real continuous-time Lyapunov matrix equation

             op(A)'*X + X*op(A) = scale*C,

      estimate the conditioning, and compute an error bound on the
      solution X, where op(A) = A or A' (A**T), the matrix A is N-by-N,
      the right hand side C and the solution X are N-by-N symmetric
      matrices (C = C', X = X'), and scale is an output scale factor,
      set less than or equal to 1 to avoid overflow in X.

Specification

          SUBROUTINE SB03TD( JOB, FACT, TRANA, UPLO, LYAPUN, N, SCALE, A,
         $                   LDA, T, LDT, U, LDU, C, LDC, X, LDX, SEP,
         $                   RCOND, FERR, WR, WI, IWORK, DWORK, LDWORK,
         $                   INFO )
    C     .. Scalar Arguments ..
          CHARACTER          FACT, JOB, LYAPUN, TRANA, UPLO
          INTEGER            INFO, LDA, LDC, LDT, LDU, LDWORK, LDX, N
          DOUBLE PRECISION   FERR, RCOND, SCALE, SEP
    C     .. Array Arguments ..
          INTEGER            IWORK( * )
          DOUBLE PRECISION   A( LDA, * ), C( LDC, * ), DWORK( * ),
         $                   T( LDT, * ), U( LDU, * ), WI( * ), WR( * ),
         $                   X( LDX, * )

Arguments

Mode Parameters

      JOB     CHARACTER*1
              Specifies the computation to be performed, as follows:
              = 'X':  Compute the solution only;
              = 'S':  Compute the separation only;
              = 'C':  Compute the reciprocal condition number only;
              = 'E':  Compute the error bound only;
              = 'A':  Compute all: the solution, separation, reciprocal
                      condition number, and the error bound.

      FACT    CHARACTER*1
              Specifies whether or not the real Schur factorization
              of the matrix A 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 A;
              = 'N':  The Schur factorization of A 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 matrix C is to be
              used, as follows:
              = 'U':  Upper triangular part;
              = 'L':  Lower triangular part.

      LYAPUN  CHARACTER*1
              Specifies whether or not the original or "reduced"
              Lyapunov equations should be solved, as follows:
              = 'O':  Solve the original Lyapunov equations, updating
                      the right-hand sides and solutions with the
                      matrix U, e.g., X <-- U'*X*U;
              = 'R':  Solve reduced Lyapunov equations only, without
                      updating the right-hand sides and solutions.
                      This means that a real Schur form T of A appears
                      in the equation, instead of A.

Input/Output Parameters

      N       (input) INTEGER
              The order of the matrices A, X, and C.  N >= 0.

      SCALE   (input or output) DOUBLE PRECISION
              If JOB = 'C' or JOB = 'E', SCALE is an input argument:
              the scale factor, set by a Lyapunov solver.
              0 <= SCALE <= 1.
              If JOB = 'X' or JOB = 'A', SCALE is an output argument:
              the scale factor, scale, set less than or equal to 1 to
              prevent the solution overflowing.
              If JOB = 'S', this argument is not used.

      A       (input) DOUBLE PRECISION array, dimension (LDA,N)
              If FACT = 'N' or (LYAPUN = 'O' and JOB <> 'X'), the
              leading N-by-N part of this array must contain the
              original matrix A.
              If FACT = 'F' and (LYAPUN = 'R' or JOB = 'X'), A is
              not referenced.

      LDA     INTEGER
              The leading dimension of the array A.
              LDA >= MAX(1,N), if FACT = 'N' or LYAPUN = 'O' and
                                                JOB <> 'X';
              LDA >= 1,        otherwise.

      T       (input/output) DOUBLE PRECISION array, dimension
              (LDT,N)
              If FACT = 'F', then 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 A.
              If FACT = 'N', then this array need not be set on input.
              On exit, (if INFO = 0 or INFO = N+1, for FACT = 'N') 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 A.
              The contents of array T is not modified if FACT = 'F'.

      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 A.
              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 A.
              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'.

      C       (input) DOUBLE PRECISION array, dimension (LDC,N)
              If JOB <> 'S' and UPLO = 'U', the leading N-by-N upper
              triangular part of this array must contain the upper
              triangular part of the matrix C of the original Lyapunov
              equation (with matrix A), if LYAPUN = 'O', or of the
              reduced Lyapunov equation (with matrix T), if
              LYAPUN = 'R'.
              If JOB <> 'S' and UPLO = 'L', the leading N-by-N lower
              triangular part of this array must contain the lower
              triangular part of the matrix C of the original Lyapunov
              equation (with matrix A), if LYAPUN = 'O', or of the
              reduced Lyapunov equation (with matrix T), if
              LYAPUN = 'R'.
              The remaining strictly triangular part of this array is
              used as workspace.
              If JOB = 'X', then this array may be identified with X
              in the call of this routine.
              If JOB = 'S', the array C is not referenced.

      LDC     INTEGER
              The leading dimension of the array C.
              LDC >= 1,        if JOB = 'S';
              LDC >= MAX(1,N), otherwise.

      X       (input or output) DOUBLE PRECISION array, dimension
              (LDX,N)
              If JOB = 'C' or 'E', then X is an input argument and on
              entry, the leading N-by-N part of this array must contain
              the symmetric solution matrix X of the original Lyapunov
              equation (with matrix A), if LYAPUN = 'O', or of the
              reduced Lyapunov equation (with matrix T), if
              LYAPUN = 'R'.
              If JOB = 'X' or 'A', then X is an output argument and on
              exit, if INFO = 0 or INFO = N+1, the leading N-by-N part
              of this array contains the symmetric solution matrix X of
              of the original Lyapunov equation (with matrix A), if
              LYAPUN = 'O', or of the reduced Lyapunov equation (with
              matrix T), if LYAPUN = 'R'.
              If JOB = 'S', the array X is not referenced.

      LDX     INTEGER
              The leading dimension of the array X.
              LDX >= 1,        if JOB = 'S';
              LDX >= MAX(1,N), otherwise.

      SEP     (output) DOUBLE PRECISION
              If JOB = 'S' or JOB = 'C' or JOB = 'A', and INFO = 0 or
              INFO = N+1, SEP contains the estimated separation of the
              matrices op(A) and -op(A)', sep(op(A),-op(A)').
              If N = 0, or X = 0, or JOB = 'X' or JOB = 'E', SEP is not
              referenced.

      RCOND   (output) DOUBLE PRECISION
              If JOB = 'C' or JOB = 'A', an estimate of the reciprocal
              condition number of the continuous-time Lyapunov equation.
              If N = 0 or X = 0, RCOND is set to 1 or 0, respectively.
              If JOB = 'X' or JOB = 'S' or JOB = 'E', RCOND is not
              referenced.

      FERR    (output) DOUBLE PRECISION
              If JOB = 'E' or JOB = 'A', and INFO = 0 or INFO = N+1,
              FERR contains an estimated forward error bound for the
              solution X. If XTRUE is the true solution, FERR bounds the
              relative error in the computed solution, measured in the
              Frobenius norm:  norm(X - XTRUE)/norm(XTRUE).
              If N = 0 or X = 0, FERR is set to 0.
              If JOB = 'X' or JOB = 'S' or JOB = 'C', FERR is not
              referenced.

      WR      (output) DOUBLE PRECISION array, dimension (N)
      WI      (output) DOUBLE PRECISION array, dimension (N)
              If FACT = 'N', and INFO = 0 or INFO = N+1, WR and WI
              contain the real and imaginary parts, respectively, of the
              eigenvalues of A.
              If FACT = 'F', WR and WI are not referenced.

Workspace

      IWORK   INTEGER array, dimension (N*N)
              This array is not referenced if JOB = 'X'.

      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 length of the array DWORK.
              If JOB = 'X', then
              LDWORK >= MAX(1,N*N),           if FACT = 'F';
              LDWORK >= MAX(1,MAX(N*N,3*N)),  if FACT = 'N'.
              If JOB = 'S' or JOB = 'C', then
              LDWORK >= MAX(1,2*N*N),         if FACT = 'F';
              LDWORK >= MAX(1,2*N*N,3*N),     if FACT = 'N'.
              If JOB = 'E', or JOB = 'A', and LYAPUN  = 'O', then
              LDWORK >= MAX(1,3*N*N);
              If JOB = 'E', or JOB = 'A', and LYAPUN  = 'R', then
              LDWORK >= MAX(1,3*N*N+N-1).
              For optimum performance LDWORK should sometimes be larger.

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 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 the elements i+1:n of WR and WI
                    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

      After reducing matrix A to real Schur canonical form (if needed),
      the Bartels-Stewart algorithm is used. A set of equivalent linear
      algebraic systems of equations of order at most four are formed
      and solved using Gaussian elimination with complete pivoting.

      The condition number of the continuous-time Lyapunov equation is
      estimated as

      cond = (norm(Theta)*norm(A) + norm(inv(Omega))*norm(C))/norm(X),

      where Omega and Theta are linear operators defined by

      Omega(W) = op(A)'*W + W*op(A),
      Theta(W) = inv(Omega(op(W)'*X + X*op(W))).

      The routine estimates the quantities

      sep(op(A),-op(A)') = 1 / norm(inv(Omega))

      and norm(Theta) using 1-norm condition estimators.

      The forward error bound is estimated using a practical error bound
      similar to the one proposed in [2].

References

      [1] Bartels, R.H. and Stewart, G.W.  T
          Solution of the matrix equation A X + XB = C.
          Comm. A.C.M., 15, pp. 820-826, 1972.

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

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.

Further Comments

      The separation of op(A) and -op(A)' can also be defined as

             sep( op(A), -op(A)' ) = sigma_min( T ),

      where sigma_min(T) is the smallest singular value of the
      N*N-by-N*N matrix

         T = kprod( I(N), op(A)' ) + kprod( op(A)', I(N) ).

      I(N) is an N-by-N identity matrix, and kprod denotes the Kronecker
      product. The routine estimates sigma_min(T) by the reciprocal of
      an estimate of the 1-norm of inverse(T). The true reciprocal
      1-norm of inverse(T) cannot differ from sigma_min(T) by more
      than a factor of N.

Example

Program Text

    *     SB03TD EXAMPLE PROGRAM TEXT
    *
    *     .. Parameters ..
          INTEGER          NIN, NOUT
          PARAMETER        ( NIN = 5, NOUT = 6 )
          INTEGER          NMAX
          PARAMETER        ( NMAX = 20 )
          INTEGER          LDA, LDC, LDT, LDU, LDX
          PARAMETER        ( LDA = NMAX, LDC = NMAX, LDT = NMAX,
         $                   LDU = NMAX, LDX = NMAX )
          INTEGER          LIWORK
          PARAMETER        ( LIWORK = NMAX*NMAX )
          INTEGER          LDWORK
          PARAMETER        ( LDWORK = MAX( 1, 3*NMAX*NMAX + NMAX - 1 ) )
    *     .. Local Scalars ..
          DOUBLE PRECISION FERR, RCOND, SCALE, SEP
          INTEGER          I, INFO, J, N
          CHARACTER*1      DICO, FACT, JOB, LYAPUN, TRANA, UPLO
    *     .. Local Arrays ..
          INTEGER          IWORK(LIWORK)
          DOUBLE PRECISION A(LDA,NMAX), C(LDC,NMAX), DWORK(LDWORK),
         $                 T(LDT,NMAX), U(LDU,NMAX), X(LDX,NMAX)
    *     .. External Functions ..
          LOGICAL          LSAME
          EXTERNAL         LSAME
    *     .. External Subroutines ..
          EXTERNAL         SB03TD
    *     .. Intrinsic Functions ..
          INTRINSIC        MAX
    *     .. Executable Statements ..
    *
          WRITE ( NOUT, FMT = 99999 )
          DICO = 'C'
    *     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 = 99994 ) N
          ELSE
             IF ( LSAME( JOB, 'C' ) .OR. LSAME( JOB, 'E' ) )
         $                               READ ( NIN, FMT = * ) SCALE
             IF ( LSAME( FACT, 'N' ) .OR. ( LSAME( LYAPUN, 'O' ) .AND.
         $                             .NOT.LSAME( JOB, 'X') ) )
         $      READ ( NIN, FMT = * ) ( ( A(I,J), J = 1,N ), I = 1,N )
             IF ( LSAME( FACT, 'F' ) ) THEN
                READ ( NIN, FMT = * ) ( ( T(I,J), J = 1,N ), I = 1,N )
                IF ( LSAME( LYAPUN, 'O' ) )
         $         READ ( NIN, FMT = * ) ( ( U(I,J), J = 1,N ), I = 1,N )
             END IF
             IF ( .NOT.LSAME( JOB, 'S' ) )
         $      READ ( NIN, FMT = * ) ( ( C(I,J), J = 1,N ), I = 1,N )
             IF ( LSAME( JOB, 'C' ) .OR. LSAME( JOB, 'E' ) )
         $      READ ( NIN, FMT = * ) ( ( X(I,J), J = 1,N ), I = 1,N )
    *        Solve the continuous-time Lyapunov matrix equation and/or
    *        estimate the condition and error bound on the solution.
             CALL SB03TD( JOB, FACT, TRANA, UPLO, LYAPUN, N, SCALE, A, LDA,
         $                T, LDT, U, LDU, C, LDC, X, LDX, SEP, RCOND, FERR,
         $                DWORK(1), DWORK(N+1), IWORK, DWORK(2*N+1),
         $                LDWORK-2*N, INFO )
    *
             IF ( INFO.EQ.0 ) THEN
                IF ( LSAME( JOB, 'X' ) .OR. LSAME( JOB, 'A' ) ) THEN
                   WRITE ( NOUT, FMT = 99996 )
                   DO 10 I = 1, N
                      WRITE ( NOUT, FMT = 99995 ) ( X(I,J), J = 1,N )
       10          CONTINUE
                   WRITE ( NOUT, FMT = 99993 ) SCALE
                END IF
                IF ( LSAME( JOB, 'S' ) .OR. LSAME( JOB, 'C' )
         $                             .OR. LSAME( JOB, 'A' ) )
         $         WRITE ( NOUT, FMT = 99992 ) SEP
                IF ( LSAME( JOB, 'C' ) .OR. LSAME( JOB, 'A' ) )
         $         WRITE ( NOUT, FMT = 99991 ) RCOND
                IF ( LSAME( JOB, 'E' ) .OR. LSAME( JOB, 'A' ) )
         $         WRITE ( NOUT, FMT = 99990 ) FERR
             ELSE
                WRITE ( NOUT, FMT = 99998 ) INFO
             END IF
          END IF
          STOP
    *
    99999 FORMAT (' SB03TD EXAMPLE PROGRAM RESULTS',/1X)
    99998 FORMAT (' INFO on exit from SB03TD =',I2)
    99996 FORMAT (' The solution matrix X is')
    99995 FORMAT (20(1X,F8.4))
    99994 FORMAT (/' N is out of range.',/' N = ',I5)
    99993 FORMAT (/' Scaling factor = ',F8.4)
    99992 FORMAT (/' Estimated separation = ',F8.4)
    99991 FORMAT (/' Estimated reciprocal condition number = ',F8.4)
    99990 FORMAT (/' Estimated error bound = ',F8.4)
          END

Program Data

     SB03TD EXAMPLE PROGRAM DATA
       3     A     N     N     U     O
       3.0   1.0   1.0
       1.0   3.0   0.0
       0.0   0.0   3.0
      25.0  24.0  15.0
      24.0  32.0   8.0
      15.0   8.0  40.0

Program Results

     SB03TD EXAMPLE PROGRAM RESULTS

     The solution matrix X is
       3.2604   2.7187   1.8616
       2.7187   4.4271   0.5699
       1.8616   0.5699   6.0461

     Scaling factor =   1.0000

     Estimated separation =   4.9068

     Estimated reciprocal condition number =   0.3611

     Estimated error bound =   0.0000

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