SB03QX

Estimating a forward error bound for the solution of a continuous-time Lyapunov equation

Purpose

  To estimate a forward error bound for the solution X of a real
  continuous-time Lyapunov matrix equation,

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

  where op(A) = A or A' (A**T) and C is symmetric (C = C**T). The
  matrix A, the right hand side C, and the solution X are N-by-N.
  An absolute residual matrix, which takes into account the rounding
  errors in forming it, is given in the array R.

      SUBROUTINE SB03QX( TRANA, UPLO, LYAPUN, N, XANORM, T, LDT, U, LDU,
     $                   R, LDR, FERR, IWORK, DWORK, LDWORK, INFO )
C     .. Scalar Arguments ..
      CHARACTER          LYAPUN, TRANA, UPLO
      INTEGER            INFO, LDR, LDT, LDU, LDWORK, N
      DOUBLE PRECISION   FERR, XANORM
C     .. Array Arguments ..
      INTEGER            IWORK( * )
      DOUBLE PRECISION   DWORK( * ), R( LDR, * ), T( LDT, * ),
     $                   U( LDU, * )

Mode Parameters

  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 R 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, 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.

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

  XANORM  (input) DOUBLE PRECISION
          The absolute (maximal) norm of the symmetric solution
          matrix X of the Lyapunov equation.  XANORM >= 0.

  T       (input) DOUBLE PRECISION array, dimension (LDT,N)
          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.

  LDT     INTEGER
          The leading dimension of array T.  LDT >= MAX(1,N).

  U       (input) DOUBLE PRECISION array, dimension (LDU,N)
          The leading N-by-N part of this array must contain the
          orthogonal matrix U from a real Schur factorization of A.
          If LYAPUN = 'R', the array U is not referenced.

  LDU     INTEGER
          The leading dimension of array U.
          LDU >= 1,        if LYAPUN = 'R';
          LDU >= MAX(1,N), if LYAPUN = 'O'.

  R       (input/output) DOUBLE PRECISION array, dimension (LDR,N)
          On entry, if UPLO = 'U', the leading N-by-N upper
          triangular part of this array must contain the upper
          triangular part of the absolute residual matrix R, with
          bounds on rounding errors added.
          On entry, if UPLO = 'L', the leading N-by-N lower
          triangular part of this array must contain the lower
          triangular part of the absolute residual matrix R, with
          bounds on rounding errors added.
          On exit, the leading N-by-N part of this array contains
          the symmetric absolute residual matrix R (with bounds on
          rounding errors added), fully stored.

  LDR     INTEGER
          The leading dimension of array R.  LDR >= MAX(1,N).

  FERR    (output) DOUBLE PRECISION
          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 XANORM = 0, FERR is set to 0, without any
          calculations.

  IWORK   INTEGER array, dimension (N*N)

  DWORK   DOUBLE PRECISION array, dimension (LDWORK)

  LDWORK  INTEGER
          The length of the array DWORK.  LDWORK >= 2*N*N.

  INFO    INTEGER
          = 0:  successful exit;
          < 0:  if INFO = -i, the i-th argument had an illegal
                value;
          = 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 is
                unchanged).

  The forward error bound is estimated using a practical error bound
  similar to the one proposed in [1], based on the 1-norm estimator
  in [2].

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

  [2] Higham, N.J.
      FORTRAN codes for estimating the one-norm of a real or
      complex matrix, with applications to condition estimation.
      ACM Trans. Math. Softw., 14, pp. 381-396, 1988.

                            3
  The algorithm requires 0(N ) operations.

  The option LYAPUN = 'R' may occasionally produce slightly worse
  or better estimates, and it is much faster than the option 'O'.
  The routine can be also used as a final step in estimating a
  forward error bound for the solution of a continuous-time
  algebraic matrix Riccati equation.

Program Text

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