# drotmg.f

          SUBROUTINE DROTMG (DD1, DD2, DX1, DY1, DPARAM)
    C***BEGIN PROLOGUE  DROTMG
    C***PURPOSE  Construct a modified Givens transformation.
    C***LIBRARY   SLATEC (BLAS)
    C***CATEGORY  D1B10
    C***TYPE      DOUBLE PRECISION (SROTMG-S, DROTMG-D)
    C***KEYWORDS  BLAS, LINEAR ALGEBRA, MODIFIED GIVENS ROTATION, VECTOR
    C***AUTHOR  Lawson, C. L., (JPL)
    C           Hanson, R. J., (SNLA)
    C           Kincaid, D. R., (U. of Texas)
    C           Krogh, F. T., (JPL)
    C***DESCRIPTION
    C
    C                B L A S  Subprogram
    C    Description of Parameters
    C
    C     --Input--
    C      DD1  double precision scalar
    C      DD2  double precision scalar
    C      DX1  double precision scalar
    C      DX2  double precision scalar
    C   DPARAM  D.P. 5-vector. DPARAM(1)=DFLAG defined below.
    C           Locations 2-5 contain the rotation matrix.
    C
    C     --Output--
    C      DD1  changed to represent the effect of the transformation
    C      DD2  changed to represent the effect of the transformation
    C      DX1  changed to represent the effect of the transformation
    C      DX2  unchanged
    C
    C     Construct the modified Givens transformation matrix H which zeros
    C     the second component of the 2-vector  (SQRT(DD1)*DX1,SQRT(DD2)*
    C     DY2)**T.
    C     With DPARAM(1)=DFLAG, H has one of the following forms:
    C
    C     DFLAG=-1.D0     DFLAG=0.D0        DFLAG=1.D0     DFLAG=-2.D0
    C
    C       (DH11  DH12)    (1.D0  DH12)    (DH11  1.D0)    (1.D0  0.D0)
    C     H=(          )    (          )    (          )    (          )
    C       (DH21  DH22),   (DH21  1.D0),   (-1.D0 DH22),   (0.D0  1.D0).
    C
    C     Locations 2-5 of DPARAM contain DH11, DH21, DH12, and DH22,
    C     respectively.  (Values of 1.D0, -1.D0, or 0.D0 implied by the
    C     value of DPARAM(1) are not stored in DPARAM.)
    C
    C***REFERENCES  C. L. Lawson, R. J. Hanson, D. R. Kincaid and F. T.
    C                 Krogh, Basic linear algebra subprograms for Fortran
    C                 usage, Algorithm No. 539, Transactions on Mathematical
    C                 Software 5, 3 (September 1979), pp. 308-323.
    C***ROUTINES CALLED  (NONE)
    C***REVISION HISTORY  (YYMMDD)
    C   780301  DATE WRITTEN
    C   890531  Changed all specific intrinsics to generic.  (WRB)
    C   890531  REVISION DATE from Version 3.2
    C   891214  Prologue converted to Version 4.0 format.  (BAB)
    C   920316  Prologue corrected.  (WRB)
    C   920501  Reformatted the REFERENCES section.  (WRB)
    C***END PROLOGUE  DROTMG