MB04OY

Applying an elementary reflector (using in-line code for a low order) to a matrix C = trans( trans(A) trans(B) ), from the left, where A has one row

Purpose

  To apply a real elementary reflector H to a real (m+1)-by-n
  matrix C = [ A ], from the left, where A has one row. H is
             [ B ]
  represented in the form
                                     ( 1 )
        H = I - tau * u *u',    u  = (   ),
                                     ( v )
  where tau is a real scalar and v is a real m-vector.

  If tau = 0, then H is taken to be the unit matrix.

  In-line code is used if H has order < 11.

      SUBROUTINE MB04OY( M, N, V, TAU, A, LDA, B, LDB, DWORK )
C     .. Scalar Arguments ..
      INTEGER           LDA, LDB, M, N
      DOUBLE PRECISION  TAU
C     .. Array Arguments ..
      DOUBLE PRECISION  A( LDA, * ), B( LDB, * ), DWORK( * ), V( * )

Input/Output Parameters

  M       (input) INTEGER
          The number of rows of the matrix B.  M >= 0.

  N       (input) INTEGER
          The number of columns of the matrices A and B.  N >= 0.

  V       (input) DOUBLE PRECISION array, dimension (M)
          The vector v in the representation of H.

  TAU     (input) DOUBLE PRECISION
          The scalar factor of the elementary reflector H.

  A       (input/output) DOUBLE PRECISION array, dimension (LDA,N)
          On entry, the leading 1-by-N part of this array must
          contain the matrix A.
          On exit, the leading 1-by-N part of this array contains
          the updated matrix A (the first row of H * C).

  LDA     INTEGER
          The leading dimension of array A.  LDA >= 1.

  B       (input/output) DOUBLE PRECISION array, dimension (LDB,N)
          On entry, the leading M-by-N part of this array must
          contain the matrix B.
          On exit, the leading M-by-N part of this array contains
          the updated matrix B (the last m rows of H * C).

  LDB     INTEGER
          The leading dimension of array B.  LDB >= MAX(1,M).

  DWORK   DOUBLE PRECISION array, dimension (N)
          DWORK is not referenced if H has order less than 11.

  The routine applies the elementary reflector H, taking the special
  structure of C into account.

  The algorithm is backward stable.

  None

Program Text

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