1 *> \brief \b SLARFG 2 * 3 * =========== DOCUMENTATION =========== 4 * 5 * Online html documentation available at 6 * http://www.netlib.org/lapack/explore-html/ 7 * 8 *> \htmlonly 9 *> Download SLARFG + dependencies 10 *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.tgz?format=tgz&filename=/lapack/lapack_routine/slarfg.f"> 11 *> [TGZ]</a> 12 *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.zip?format=zip&filename=/lapack/lapack_routine/slarfg.f"> 13 *> [ZIP]</a> 14 *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.txt?format=txt&filename=/lapack/lapack_routine/slarfg.f"> 15 *> [TXT]</a> 16 *> \endhtmlonly 17 * 18 * Definition: 19 * =========== 20 * 21 * SUBROUTINE SLARFG( N, ALPHA, X, INCX, TAU ) 22 * 23 * .. Scalar Arguments .. 24 * INTEGER INCX, N 25 * REAL ALPHA, TAU 26 * .. 27 * .. Array Arguments .. 28 * REAL X( * ) 29 * .. 30 * 31 * 32 *> \par Purpose: 33 * ============= 34 *> 35 *> \verbatim 36 *> 37 *> SLARFG 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 REAL 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 REAL 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 REAL 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 realOTHERauxiliary 105 * 106 * ===================================================================== 107 SUBROUTINE SLARFG( 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 REAL ALPHA, TAU 117 * .. 118 * .. Array Arguments .. 119 REAL X( * ) 120 * .. 121 * 122 * ===================================================================== 123 * 124 * .. Parameters .. 125 REAL ONE, ZERO 126 PARAMETER ( ONE = 1.0E+0, ZERO = 0.0E+0 ) 127 * .. 128 * .. Local Scalars .. 129 INTEGER J, KNT 130 REAL BETA, RSAFMN, SAFMIN, XNORM 131 * .. 132 * .. External Functions .. 133 REAL SLAMCH, SLAPY2, SNRM2 134 EXTERNAL SLAMCH, SLAPY2, SNRM2 135 * .. 136 * .. Intrinsic Functions .. 137 INTRINSIC ABS, SIGN 138 * .. 139 * .. External Subroutines .. 140 EXTERNAL SSCAL 141 * .. 142 * .. Executable Statements .. 143 * 144 IF( N.LE.1 ) THEN 145 TAU = ZERO 146 RETURN 147 END IF 148 * 149 XNORM = SNRM2( 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( SLAPY2( ALPHA, XNORM ), ALPHA ) 161 SAFMIN = SLAMCH( 'S' ) / SLAMCH( '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 SSCAL( 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 = SNRM2( N-1, X, INCX ) 179 BETA = -SIGN( SLAPY2( ALPHA, XNORM ), ALPHA ) 180 END IF 181 TAU = ( BETA-ALPHA ) / BETA 182 CALL SSCAL( 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 SLARFG 195 * 196 END 197
