dlatdf.f(3)

NAME
dlatdf.f -

SYNOPSIS

  Functions/Subroutines

subroutine dlatdf (IJOB, N, Z, LDZ, RHS, RDSUM, RDSCAL, IPIV, JPIV)
DLATDF uses the LU factorization of the n-by-n matrix computed by sgetc2 and computes a contribution to the reciprocal Dif-estimate.

Function/Subroutine Documentation

  subroutine dlatdf (integerIJOB, integerN, double precision, dimension( ldz, * )Z, integerLDZ, double precision, dimension( * )RHS, double precisionRDSUM, double precisionRDSCAL, integer, dimension( * )IPIV, integer, dimension( * )JPIV)

DLATDF uses the LU factorization of the n-by-n matrix computed by sgetc2 and computes a contribution to the reciprocal Dif-estimate.

Purpose:

     DLATDF uses the LU factorization of the n-by-n matrix Z computed by
     DGETC2 and computes a contribution to the reciprocal Dif-estimate
     by solving Z * x = b for x, and choosing the r.h.s. b such that
     the norm of x is as large as possible. On entry RHS = b holds the
     contribution from earlier solved sub-systems, and on return RHS = x.

     The factorization of Z returned by DGETC2 has the form Z = P*L*U*Q,
     where P and Q are permutation matrices. L is lower triangular with
     unit diagonal elements and U is upper triangular.

Parameters:
    IJOB

              IJOB is INTEGER
              IJOB = 2: First compute an approximative null-vector e
                  of Z using DGECON, e is normalized and solve for
                  Zx = +-e - f with the sign giving the greater value
                  of 2-norm(x). About 5 times as expensive as Default.
              IJOB .ne. 2: Local look ahead strategy where all entries of
                  the r.h.s. b is choosen as either +1 or -1 (Default).

    N

              N is INTEGER
              The number of columns of the matrix Z.

    Z

              Z is DOUBLE PRECISION array, dimension (LDZ, N)
              On entry, the LU part of the factorization of the n-by-n
              matrix Z computed by DGETC2:  Z = P * L * U * Q

    LDZ

              LDZ is INTEGER
              The leading dimension of the array Z.  LDA >= max(1, N).

    RHS

              RHS is DOUBLE PRECISION array, dimension (N)
              On entry, RHS contains contributions from other subsystems.
              On exit, RHS contains the solution of the subsystem with
              entries acoording to the value of IJOB (see above).

    RDSUM

              RDSUM is DOUBLE PRECISION
              On entry, the sum of squares of computed contributions to
              the Dif-estimate under computation by DTGSYL, where the
              scaling factor RDSCAL (see below) has been factored out.
              On exit, the corresponding sum of squares updated with the
              contributions from the current sub-system.
              If TRANS = 'T' RDSUM is not touched.
              NOTE: RDSUM only makes sense when DTGSY2 is called by STGSYL.

    RDSCAL

              RDSCAL is DOUBLE PRECISION
              On entry, scaling factor used to prevent overflow in RDSUM.
              On exit, RDSCAL is updated w.r.t. the current contributions
              in RDSUM.
              If TRANS = 'T', RDSCAL is not touched.
              NOTE: RDSCAL only makes sense when DTGSY2 is called by
                    DTGSYL.

    IPIV

              IPIV is INTEGER array, dimension (N).
              The pivot indices; for 1 <= i <= N, row i of the
              matrix has been interchanged with row IPIV(i).

    JPIV

              JPIV is INTEGER array, dimension (N).
              The pivot indices; for 1 <= j <= N, column j of the
              matrix has been interchanged with column JPIV(j).

Author:
    Univ. of Tennessee

    Univ. of California Berkeley

    Univ. of Colorado Denver

    NAG Ltd.

Date:
    September 2012

Further Details:
    This routine is a further developed implementation of algorithm BSOLVE in [1] using complete pivoting in the LU factorization.

Contributors:
    Bo Kagstrom and Peter Poromaa, Department of Computing Science, Umea University, S-901 87 Umea, Sweden.

References:

      [1] Bo Kagstrom and Lars Westin,
          Generalized Schur Methods with Condition Estimators for
          Solving the Generalized Sylvester Equation, IEEE Transactions
          on Automatic Control, Vol. 34, No. 7, July 1989, pp 745-751.

      [2] Peter Poromaa,
          On Efficient and Robust Estimators for the Separation
          between two Regular Matrix Pairs with Applications in
          Condition Estimation. Report IMINF-95.05, Departement of
          Computing Science, Umea University, S-901 87 Umea, Sweden, 1995.

Definition at line 171 of file dlatdf.f.

Author

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