clarfg()

subroutine clarfg	(	integer	n,
		complex	alpha,
		complex, dimension( * )	x,
		integer	incx,
		complex	tau )

CLARFG generates an elementary reflector (Householder matrix).

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Purpose:

!>
!> CLARFG generates a complex elementary reflector H of order n, such
!> that
!>
!>       H**H * ( alpha ) = ( beta ),   H**H * H = I.
!>              (   x   )   (   0  )
!>
!> where alpha and beta are scalars, with beta real, and x is an
!> (n-1)-element complex vector. H is represented in the form
!>
!>       H = I - tau * ( 1 ) * ( 1 v**H ) ,
!>                     ( v )
!>
!> where tau is a complex scalar and v is a complex (n-1)-element
!> vector. Note that H is not hermitian.
!>
!> If the elements of x are all zero and alpha is real, then tau = 0
!> and H is taken to be the unit matrix.
!>
!> Otherwise  1 <= real(tau) <= 2  and  abs(tau-1) <= 1 .
!> 

Parameters

[in]	N	!> N is INTEGER !> The order of the elementary reflector. !>
[in,out]	ALPHA	!> ALPHA is COMPLEX !> On entry, the value alpha. !> On exit, it is overwritten with the value beta. !>
[in,out]	X	!> X is COMPLEX array, dimension !> (1+(N-2)*abs(INCX)) !> On entry, the vector x. !> On exit, it is overwritten with the vector v. !>
[in]	INCX	!> INCX is INTEGER !> The increment between elements of X. INCX > 0. !>
[out]	TAU	!> TAU is COMPLEX !> The value tau. !>

Author

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

Definition at line 103 of file clarfg.f.

*
*  -- 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
      COMPLEX            ALPHA, TAU
*     ..
*     .. Array Arguments ..
      COMPLEX            X( * )
*     ..
*
*  =====================================================================
*
*     .. Parameters ..
      REAL               ONE, ZERO
      parameter( one = 1.0e+0, zero = 0.0e+0 )
*     ..
*     .. Local Scalars ..
      INTEGER            J, KNT
      REAL               ALPHI, ALPHR, BETA, RSAFMN, SAFMIN, XNORM
*     ..
*     .. External Functions ..
      REAL               SCNRM2, SLAMCH, SLAPY3
      COMPLEX            CLADIV
      EXTERNAL           scnrm2, slamch, slapy3, cladiv
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          abs, aimag, cmplx, real, sign
*     ..
*     .. External Subroutines ..
      EXTERNAL           cscal, csscal
*     ..
*     .. Executable Statements ..
*
      IF( n.LE.0 ) THEN
         tau = zero
         RETURN
      END IF
*
      xnorm = scnrm2( n-1, x, incx )
      alphr = real( alpha )
      alphi = aimag( alpha )
*
      IF( xnorm.EQ.zero .AND. alphi.EQ.zero ) THEN
*
*        H  =  I
*
         tau = zero
      ELSE
*
*        general case
*
         beta = -sign( slapy3( alphr, alphi, xnorm ), alphr )
         safmin = slamch( 'S' ) / slamch( 'E' )
         rsafmn = one / safmin
*
         knt = 0
         IF( abs( beta ).LT.safmin ) THEN
*
*           XNORM, BETA may be inaccurate; scale X and recompute them
*
   10       CONTINUE
            knt = knt + 1
            CALL csscal( n-1, rsafmn, x, incx )
            beta = beta*rsafmn
            alphi = alphi*rsafmn
            alphr = alphr*rsafmn
            IF( (abs( beta ).LT.safmin) .AND. (knt .LT. 20) )
     $         GO TO 10
*
*           New BETA is at most 1, at least SAFMIN
*
            xnorm = scnrm2( n-1, x, incx )
            alpha = cmplx( alphr, alphi )
            beta = -sign( slapy3( alphr, alphi, xnorm ), alphr )
         END IF
         tau = cmplx( ( beta-alphr ) / beta, -alphi / beta )
         alpha = cladiv( cmplx( one ), alpha-beta )
         CALL cscal( n-1, alpha, 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 CLARFG
*

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