slartgs.f

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*> \brief \b SLARTGS generates a plane rotation designed to introduce a bulge in implicit QR iteration for the bidiagonal SVD problem.
*
*  =========== DOCUMENTATION ===========
*
* Online html documentation available at 
*            http://www.netlib.org/lapack/explore-html/ 
*
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*> \endhtmlonly 
*
*  Definition:
*  ===========
*
*       SUBROUTINE SLARTGS( X, Y, SIGMA, CS, SN )
* 
*       .. Scalar Arguments ..
*       REAL                    CS, SIGMA, SN, X, Y
*       ..
*  
*
*> \par Purpose:
*  =============
*>
*> \verbatim
*>
*> SLARTGS generates a plane rotation designed to introduce a bulge in
*> Golub-Reinsch-style implicit QR iteration for the bidiagonal SVD
*> problem. X and Y are the top-row entries, and SIGMA is the shift.
*> The computed CS and SN define a plane rotation satisfying
*>
*>    [  CS  SN  ]  .  [ X^2 - SIGMA ]  =  [ R ],
*>    [ -SN  CS  ]     [    X * Y    ]     [ 0 ]
*>
*> with R nonnegative.  If X^2 - SIGMA and X * Y are 0, then the
*> rotation is by PI/2.
*> \endverbatim
*
*  Arguments:
*  ==========
*
*> \param[in] X
*> \verbatim
*>          X is REAL
*>          The (1,1) entry of an upper bidiagonal matrix.
*> \endverbatim
*>
*> \param[in] Y
*> \verbatim
*>          Y is REAL
*>          The (1,2) entry of an upper bidiagonal matrix.
*> \endverbatim
*>
*> \param[in] SIGMA
*> \verbatim
*>          SIGMA is REAL
*>          The shift.
*> \endverbatim
*>
*> \param[out] CS
*> \verbatim
*>          CS is REAL
*>          The cosine of the rotation.
*> \endverbatim
*>
*> \param[out] SN
*> \verbatim
*>          SN is REAL
*>          The sine of the rotation.
*> \endverbatim
*
*  Authors:
*  ========
*
*> \author Univ. of Tennessee 
*> \author Univ. of California Berkeley 
*> \author Univ. of Colorado Denver 
*> \author NAG Ltd. 
*
*> \date September 2012
*
*> \ingroup auxOTHERcomputational
*
*  =====================================================================
      SUBROUTINE slartgs( X, Y, SIGMA, CS, SN )
*
*  -- LAPACK computational routine (version 3.4.2) --
*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*     September 2012
*
*     .. Scalar Arguments ..
      REAL                    cs, sigma, sn, x, y
*     ..
*
*  ===================================================================
*
*     .. Parameters ..
      REAL                    negone, one, zero
      parameter( negone = -1.0e0, one = 1.0e0, zero = 0.0e0 )
*     ..
*     .. Local Scalars ..
      REAL                    r, s, thresh, w, z
*     ..
*     .. External Functions ..
      REAL                    slamch
      EXTERNAL           slamch
*     .. Executable Statements ..
*
      thresh = slamch('E')
*
*     Compute the first column of B**T*B - SIGMA^2*I, up to a scale
*     factor.
*
      IF( (sigma .EQ. zero .AND. abs(x) .LT. thresh) .OR.
     $          (abs(x) .EQ. sigma .AND. y .EQ. zero) ) THEN
         z = zero
         w = zero
      ELSE IF( sigma .EQ. zero ) THEN
         IF( x .GE. zero ) THEN
            z = x
            w = y
         ELSE
            z = -x
            w = -y
         END IF
      ELSE IF( abs(x) .LT. thresh ) THEN
         z = -sigma*sigma
         w = zero
      ELSE
         IF( x .GE. zero ) THEN
            s = one
         ELSE
            s = negone
         END IF
         z = s * (abs(x)-sigma) * (s+sigma/x)
         w = s * y
      END IF
*
*     Generate the rotation.
*     CALL SLARTGP( Z, W, CS, SN, R ) might seem more natural;
*     reordering the arguments ensures that if Z = 0 then the rotation
*     is by PI/2.
*
      CALL slartgp( w, z, sn, cs, r )
*
      return
*
*     End SLARTGS
*
      END