ground/gcs/src/libs/eigen/lapack/dlarfg.f

   1 *> \brief \b DLARFG
   2 *
   3 *  =========== DOCUMENTATION ===========
   4 *
   5 * Online html documentation available at
   6 *            http://www.netlib.org/lapack/explore-html/
   7 *
   8 *> \htmlonly
   9 *> Download DLARFG + dependencies
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  11 *> [TGZ]</a>
  12 *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.zip?format=zip&filename=/lapack/lapack_routine/dlarfg.f">
  13 *> [ZIP]</a>
  14 *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.txt?format=txt&filename=/lapack/lapack_routine/dlarfg.f">
  15 *> [TXT]</a>
  16 *> \endhtmlonly
  17 *
  18 *  Definition:
  19 *  ===========
  20 *
  21 *       SUBROUTINE DLARFG( N, ALPHA, X, INCX, TAU )
  22 *
  23 *       .. Scalar Arguments ..
  24 *       INTEGER            INCX, N
  25 *       DOUBLE PRECISION   ALPHA, TAU
  26 *       ..
  27 *       .. Array Arguments ..
  28 *       DOUBLE PRECISION   X( * )
  29 *       ..
  30 *
  31 *
  32 *> \par Purpose:
  33 *  =============
  34 *>
  35 *> \verbatim
  36 *>
  37 *> DLARFG generates a real elementary reflector H of order n, such
  38 *> that
  39 *>
  40 *>       H * ( alpha ) = ( beta ),   H**T * H = I.
  41 *>           (   x   )   (   0  )
  42 *>
  43 *> where alpha and beta are scalars, and x is an (n-1)-element real
  44 *> vector. H is represented in the form
  45 *>
  46 *>       H = I - tau * ( 1 ) * ( 1 v**T ) ,
  47 *>                     ( v )
  48 *>
  49 *> where tau is a real scalar and v is a real (n-1)-element
  50 *> vector.
  51 *>
  52 *> If the elements of x are all zero, then tau = 0 and H is taken to be
  53 *> the unit matrix.
  54 *>
  55 *> Otherwise  1 <= tau <= 2.
  56 *> \endverbatim
  57 *
  58 *  Arguments:
  59 *  ==========
  60 *
  61 *> \param[in] N
  62 *> \verbatim
  63 *>          N is INTEGER
  64 *>          The order of the elementary reflector.
  65 *> \endverbatim
  66 *>
  67 *> \param[in,out] ALPHA
  68 *> \verbatim
  69 *>          ALPHA is DOUBLE PRECISION
  70 *>          On entry, the value alpha.
  71 *>          On exit, it is overwritten with the value beta.
  72 *> \endverbatim
  73 *>
  74 *> \param[in,out] X
  75 *> \verbatim
  76 *>          X is DOUBLE PRECISION array, dimension
  77 *>                         (1+(N-2)*abs(INCX))
  78 *>          On entry, the vector x.
  79 *>          On exit, it is overwritten with the vector v.
  80 *> \endverbatim
  81 *>
  82 *> \param[in] INCX
  83 *> \verbatim
  84 *>          INCX is INTEGER
  85 *>          The increment between elements of X. INCX > 0.
  86 *> \endverbatim
  87 *>
  88 *> \param[out] TAU
  89 *> \verbatim
  90 *>          TAU is DOUBLE PRECISION
  91 *>          The value tau.
  92 *> \endverbatim
  93 *
  94 *  Authors:
  95 *  ========
  96 *
  97 *> \author Univ. of Tennessee
  98 *> \author Univ. of California Berkeley
  99 *> \author Univ. of Colorado Denver
 100 *> \author NAG Ltd.
 101 *
 102 *> \date November 2011
 103 *
 104 *> \ingroup doubleOTHERauxiliary
 105 *
 106 *  =====================================================================
 107       SUBROUTINE DLARFG( N, ALPHA, X, INCX, TAU )
 108 *
 109 *  -- LAPACK auxiliary routine (version 3.4.0) --
 110 *  -- LAPACK is a software package provided by Univ. of Tennessee,    --
 111 *  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
 112 *     November 2011
 113 *
 114 *     .. Scalar Arguments ..
 115       INTEGER            INCX, N
 116       DOUBLE PRECISION   ALPHA, TAU
 117 *     ..
 118 *     .. Array Arguments ..
 119       DOUBLE PRECISION   X( * )
 120 *     ..
 121 *
 122 *  =====================================================================
 123 *
 124 *     .. Parameters ..
 125       DOUBLE PRECISION   ONE, ZERO
 126       PARAMETER          ( ONE = 1.0D+0, ZERO = 0.0D+0 )
 127 *     ..
 128 *     .. Local Scalars ..
 129       INTEGER            J, KNT
 130       DOUBLE PRECISION   BETA, RSAFMN, SAFMIN, XNORM
 131 *     ..
 132 *     .. External Functions ..
 133       DOUBLE PRECISION   DLAMCH, DLAPY2, DNRM2
 134       EXTERNAL           DLAMCH, DLAPY2, DNRM2
 135 *     ..
 136 *     .. Intrinsic Functions ..
 137       INTRINSIC          ABS, SIGN
 138 *     ..
 139 *     .. External Subroutines ..
 140       EXTERNAL           DSCAL
 141 *     ..
 142 *     .. Executable Statements ..
 143 *
 144       IF( N.LE.1 ) THEN
 145          TAU = ZERO
 146          RETURN
 147       END IF
 148 *
 149       XNORM = DNRM2( N-1, X, INCX )
 150 *
 151       IF( XNORM.EQ.ZERO ) THEN
 152 *
 153 *        H  =  I
 154 *
 155          TAU = ZERO
 156       ELSE
 157 *
 158 *        general case
 159 *
 160          BETA = -SIGN( DLAPY2( ALPHA, XNORM ), ALPHA )
 161          SAFMIN = DLAMCH( 'S' ) / DLAMCH( 'E' )
 162          KNT = 0
 163          IF( ABS( BETA ).LT.SAFMIN ) THEN
 164 *
 165 *           XNORM, BETA may be inaccurate; scale X and recompute them
 166 *
 167             RSAFMN = ONE / SAFMIN
 168    10       CONTINUE
 169             KNT = KNT + 1
 170             CALL DSCAL( N-1, RSAFMN, X, INCX )
 171             BETA = BETA*RSAFMN
 172             ALPHA = ALPHA*RSAFMN
 173             IF( ABS( BETA ).LT.SAFMIN )
 174      $         GO TO 10
 175 *
 176 *           New BETA is at most 1, at least SAFMIN
 177 *
 178             XNORM = DNRM2( N-1, X, INCX )
 179             BETA = -SIGN( DLAPY2( ALPHA, XNORM ), ALPHA )
 180          END IF
 181          TAU = ( BETA-ALPHA ) / BETA
 182          CALL DSCAL( N-1, ONE / ( ALPHA-BETA ), X, INCX )
 183 *
 184 *        If ALPHA is subnormal, it may lose relative accuracy
 185 *
 186          DO 20 J = 1, KNT
 187             BETA = BETA*SAFMIN
 188  20      CONTINUE
 189          ALPHA = BETA
 190       END IF
 191 *
 192       RETURN
 193 *
 194 *     End of DLARFG
 195 *
 196       END