SUBROUTINE iau_RX ( PHI, R )
*+
*  - - - - - - -
*   i a u _ R X
*  - - - - - - -
*
*  Rotate an r-matrix about the x-axis.
*
*  This routine is part of the International Astronomical Union's
*  SOFA (Standards of Fundamental Astronomy) software collection.
*
*  Status:  vector/matrix support routine.
*
*  Given:
*     PHI      d         angle (radians)
*
*  Given and returned:
*     R        d(3,3)    r-matrix, rotated
*
*  Notes:
*
*  1) Calling this routine with positive PHI incorporates in the
*     supplied r-matrix R an additional rotation, about the x-axis,
*     anticlockwise as seen looking towards the origin from positive x.
*
*  2) The additional rotation can be represented by this matrix:
*
*         (  1        0            0      )
*         (                               )
*         (  0   + cos(PHI)   + sin(PHI)  )
*         (                               )
*         (  0   - sin(PHI)   + cos(PHI)  )
*
*  This revision:  2012 April 3
*
*  SOFA release 2020-07-21
*
*  Copyright (C) 2020 IAU SOFA Board.  See notes at end.
*
*-----------------------------------------------------------------------

      IMPLICIT NONE

      DOUBLE PRECISION PHI, R(3,3)

      DOUBLE PRECISION S, C, A21, A22, A23, A31, A32, A33

* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

      S = SIN(PHI)
      C = COS(PHI)

      A21 =   C*R(2,1) + S*R(3,1)
      A22 =   C*R(2,2) + S*R(3,2)
      A23 =   C*R(2,3) + S*R(3,3)
      A31 = - S*R(2,1) + C*R(3,1)
      A32 = - S*R(2,2) + C*R(3,2)
      A33 = - S*R(2,3) + C*R(3,3)

      R(2,1) = A21
      R(2,2) = A22
      R(2,3) = A23
      R(3,1) = A31
      R(3,2) = A32
      R(3,3) = A33

*  Finished.

*+----------------------------------------------------------------------
*
*  Copyright (C) 2020
*  Standards Of Fundamental Astronomy Board
*  of the International Astronomical Union.
*
*  =====================
*  SOFA Software License
*  =====================
*
*  NOTICE TO USER:
*
*  BY USING THIS SOFTWARE YOU ACCEPT THE FOLLOWING SIX TERMS AND
*  CONDITIONS WHICH APPLY TO ITS USE.
*
*  1. The Software is owned by the IAU SOFA Board ("SOFA").
*
*  2. Permission is granted to anyone to use the SOFA software for any
*     purpose, including commercial applications, free of charge and
*     without payment of royalties, subject to the conditions and
*     restrictions listed below.
*
*  3. You (the user) may copy and distribute SOFA source code to others,
*     and use and adapt its code and algorithms in your own software,
*     on a world-wide, royalty-free basis.  That portion of your
*     distribution that does not consist of intact and unchanged copies
*     of SOFA source code files is a "derived work" that must comply
*     with the following requirements:
*
*     a) Your work shall be marked or carry a statement that it
*        (i) uses routines and computations derived by you from
*        software provided by SOFA under license to you; and
*        (ii) does not itself constitute software provided by and/or
*        endorsed by SOFA.
*
*     b) The source code of your derived work must contain descriptions
*        of how the derived work is based upon, contains and/or differs
*        from the original SOFA software.
*
*     c) The names of all routines in your derived work shall not
*        include the prefix "iau" or "sofa" or trivial modifications
*        thereof such as changes of case.
*
*     d) The origin of the SOFA components of your derived work must
*        not be misrepresented;  you must not claim that you wrote the
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*
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*
*     Note that, as originally distributed, the SOFA software is
*     intended to be a definitive implementation of the IAU standards,
*     and consequently third-party modifications are discouraged.  All
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*
*  4. You shall not cause the SOFA software to be brought into
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*
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*     SOFA representative has been advised of such damages, or for any
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*
*  6. The provision of any version of the SOFA software under the terms
*     and conditions specified herein does not imply that future
*     versions will also be made available under the same terms and
*     conditions.
*
*  In any published work or commercial product which uses the SOFA
*  software directly, acknowledgement (see www.iausofa.org) is
*  appreciated.
*
*  Correspondence concerning SOFA software should be addressed as
*  follows:
*
*      By email:  sofa@ukho.gov.uk
*      By post:   IAU SOFA Center
*                 HM Nautical Almanac Office
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*
*-----------------------------------------------------------------------

      END