slarfg.f

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*> \brief \b SLARFG generates an elementary reflector (Householder matrix).
*
*  =========== DOCUMENTATION ===========
*
* Online html documentation available at
*            http://www.netlib.org/lapack/explore-html/
*
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*
*  Definition:
*  ===========
*
*       SUBROUTINE SLARFG( N, ALPHA, X, INCX, TAU )
*
*       .. Scalar Arguments ..
*       INTEGER            INCX, N
*       REAL               ALPHA, TAU
*       ..
*       .. Array Arguments ..
*       REAL               X( * )
*       ..
*
*
*> \par Purpose:
*  =============
*>
*> \verbatim
*>
*> SLARFG generates a real elementary reflector H of order n, such
*> that
*>
*>       H * ( alpha ) = ( beta ),   H**T * H = I.
*>           (   x   )   (   0  )
*>
*> where alpha and beta are scalars, and x is an (n-1)-element real
*> vector. H is represented in the form
*>
*>       H = I - tau * ( 1 ) * ( 1 v**T ) ,
*>                     ( v )
*>
*> where tau is a real scalar and v is a real (n-1)-element
*> vector.
*>
*> If the elements of x are all zero, then tau = 0 and H is taken to be
*> the unit matrix.
*>
*> Otherwise  1 <= tau <= 2.
*> \endverbatim
*
*  Arguments:
*  ==========
*
*> \param[in] N
*> \verbatim
*>          N is INTEGER
*>          The order of the elementary reflector.
*> \endverbatim
*>
*> \param[in,out] ALPHA
*> \verbatim
*>          ALPHA is REAL
*>          On entry, the value alpha.
*>          On exit, it is overwritten with the value beta.
*> \endverbatim
*>
*> \param[in,out] X
*> \verbatim
*>          X is REAL array, dimension
*>                         (1+(N-2)*abs(INCX))
*>          On entry, the vector x.
*>          On exit, it is overwritten with the vector v.
*> \endverbatim
*>
*> \param[in] INCX
*> \verbatim
*>          INCX is INTEGER
*>          The increment between elements of X. INCX > 0.
*> \endverbatim
*>
*> \param[out] TAU
*> \verbatim
*>          TAU is REAL
*>          The value tau.
*> \endverbatim
*
*  Authors:
*  ========
*
*> \author Univ. of Tennessee
*> \author Univ. of California Berkeley
*> \author Univ. of Colorado Denver
*> \author NAG Ltd.
*
*> \ingroup larfg
*
*  =====================================================================
      SUBROUTINE slarfg( N, ALPHA, X, INCX, TAU )
*
*  -- LAPACK auxiliary routine --
*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*
*     .. Scalar Arguments ..
      INTEGER            INCX, N
      REAL               ALPHA, TAU
*     ..
*     .. Array Arguments ..
      REAL               X( * )
*     ..
*
*  =====================================================================
*
*     .. Parameters ..
      REAL               ONE, ZERO
      parameter( one = 1.0e+0, zero = 0.0e+0 )
*     ..
*     .. Local Scalars ..
      INTEGER            J, KNT
      REAL               BETA, RSAFMN, SAFMIN, XNORM
*     ..
*     .. External Functions ..
      REAL               SLAMCH, SLAPY2, SNRM2
      EXTERNAL           slamch, slapy2, snrm2
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          abs, sign
*     ..
*     .. External Subroutines ..
      EXTERNAL           sscal
*     ..
*     .. Executable Statements ..
*
      IF( n.LE.1 ) THEN
         tau = zero
         RETURN
      END IF
*
      xnorm = snrm2( n-1, x, incx )
*
      IF( xnorm.EQ.zero ) THEN
*
*        H  =  I
*
         tau = zero
      ELSE
*
*        general case
*
         beta = -sign( slapy2( alpha, xnorm ), alpha )
         safmin = slamch( 'S' ) / slamch( 'E' )
         knt = 0
         IF( abs( beta ).LT.safmin ) THEN
*
*           XNORM, BETA may be inaccurate; scale X and recompute them
*
            rsafmn = one / safmin
   10       CONTINUE
            knt = knt + 1
            CALL sscal( n-1, rsafmn, x, incx )
            beta = beta*rsafmn
            alpha = alpha*rsafmn
            IF( (abs( beta ).LT.safmin) .AND. (knt .LT. 20) )
     $         GO TO 10
*
*           New BETA is at most 1, at least SAFMIN
*
            xnorm = snrm2( n-1, x, incx )
            beta = -sign( slapy2( alpha, xnorm ), alpha )
         END IF
         tau = ( beta-alpha ) / beta
         CALL sscal( n-1, one / ( alpha-beta ), x, incx )
*
*        If ALPHA is subnormal, it may lose relative accuracy
*
         DO 20 j = 1, knt
            beta = beta*safmin
 20      CONTINUE
         alpha = beta
      END IF
*
      RETURN
*
*     End of SLARFG
*
      END