1 | *> \brief \b SLARZT |
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2 | * |
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3 | * =========== DOCUMENTATION =========== |
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4 | * |
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5 | * Online html documentation available at |
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6 | * http://www.netlib.org/lapack/explore-html/ |
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7 | * |
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8 | *> \htmlonly |
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9 | *> Download SLARZT + dependencies |
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10 | *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.tgz?format=tgz&filename=/lapack/lapack_routine/slarzt.f"> |
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11 | *> [TGZ]</a> |
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12 | *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.zip?format=zip&filename=/lapack/lapack_routine/slarzt.f"> |
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13 | *> [ZIP]</a> |
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14 | *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.txt?format=txt&filename=/lapack/lapack_routine/slarzt.f"> |
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15 | *> [TXT]</a> |
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16 | *> \endhtmlonly |
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17 | * |
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18 | * Definition: |
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19 | * =========== |
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20 | * |
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21 | * SUBROUTINE SLARZT( DIRECT, STOREV, N, K, V, LDV, TAU, T, LDT ) |
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22 | * |
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23 | * .. Scalar Arguments .. |
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24 | * CHARACTER DIRECT, STOREV |
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25 | * INTEGER K, LDT, LDV, N |
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26 | * .. |
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27 | * .. Array Arguments .. |
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28 | * REAL T( LDT, * ), TAU( * ), V( LDV, * ) |
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29 | * .. |
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30 | * |
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31 | * |
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32 | *> \par Purpose: |
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33 | * ============= |
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34 | *> |
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35 | *> \verbatim |
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36 | *> |
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37 | *> SLARZT forms the triangular factor T of a real block reflector |
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38 | *> H of order > n, which is defined as a product of k elementary |
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39 | *> reflectors. |
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40 | *> |
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41 | *> If DIRECT = 'F', H = H(1) H(2) . . . H(k) and T is upper triangular; |
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42 | *> |
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43 | *> If DIRECT = 'B', H = H(k) . . . H(2) H(1) and T is lower triangular. |
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44 | *> |
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45 | *> If STOREV = 'C', the vector which defines the elementary reflector |
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46 | *> H(i) is stored in the i-th column of the array V, and |
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47 | *> |
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48 | *> H = I - V * T * V**T |
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49 | *> |
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50 | *> If STOREV = 'R', the vector which defines the elementary reflector |
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51 | *> H(i) is stored in the i-th row of the array V, and |
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52 | *> |
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53 | *> H = I - V**T * T * V |
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54 | *> |
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55 | *> Currently, only STOREV = 'R' and DIRECT = 'B' are supported. |
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56 | *> \endverbatim |
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57 | * |
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58 | * Arguments: |
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59 | * ========== |
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60 | * |
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61 | *> \param[in] DIRECT |
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62 | *> \verbatim |
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63 | *> DIRECT is CHARACTER*1 |
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64 | *> Specifies the order in which the elementary reflectors are |
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65 | *> multiplied to form the block reflector: |
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66 | *> = 'F': H = H(1) H(2) . . . H(k) (Forward, not supported yet) |
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67 | *> = 'B': H = H(k) . . . H(2) H(1) (Backward) |
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68 | *> \endverbatim |
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69 | *> |
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70 | *> \param[in] STOREV |
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71 | *> \verbatim |
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72 | *> STOREV is CHARACTER*1 |
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73 | *> Specifies how the vectors which define the elementary |
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74 | *> reflectors are stored (see also Further Details): |
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75 | *> = 'C': columnwise (not supported yet) |
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76 | *> = 'R': rowwise |
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77 | *> \endverbatim |
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78 | *> |
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79 | *> \param[in] N |
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80 | *> \verbatim |
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81 | *> N is INTEGER |
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82 | *> The order of the block reflector H. N >= 0. |
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83 | *> \endverbatim |
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84 | *> |
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85 | *> \param[in] K |
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86 | *> \verbatim |
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87 | *> K is INTEGER |
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88 | *> The order of the triangular factor T (= the number of |
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89 | *> elementary reflectors). K >= 1. |
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90 | *> \endverbatim |
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91 | *> |
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92 | *> \param[in,out] V |
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93 | *> \verbatim |
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94 | *> V is REAL array, dimension |
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95 | *> (LDV,K) if STOREV = 'C' |
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96 | *> (LDV,N) if STOREV = 'R' |
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97 | *> The matrix V. See further details. |
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98 | *> \endverbatim |
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99 | *> |
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100 | *> \param[in] LDV |
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101 | *> \verbatim |
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102 | *> LDV is INTEGER |
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103 | *> The leading dimension of the array V. |
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104 | *> If STOREV = 'C', LDV >= max(1,N); if STOREV = 'R', LDV >= K. |
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105 | *> \endverbatim |
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106 | *> |
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107 | *> \param[in] TAU |
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108 | *> \verbatim |
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109 | *> TAU is REAL array, dimension (K) |
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110 | *> TAU(i) must contain the scalar factor of the elementary |
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111 | *> reflector H(i). |
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112 | *> \endverbatim |
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113 | *> |
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114 | *> \param[out] T |
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115 | *> \verbatim |
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116 | *> T is REAL array, dimension (LDT,K) |
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117 | *> The k by k triangular factor T of the block reflector. |
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118 | *> If DIRECT = 'F', T is upper triangular; if DIRECT = 'B', T is |
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119 | *> lower triangular. The rest of the array is not used. |
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120 | *> \endverbatim |
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121 | *> |
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122 | *> \param[in] LDT |
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123 | *> \verbatim |
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124 | *> LDT is INTEGER |
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125 | *> The leading dimension of the array T. LDT >= K. |
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126 | *> \endverbatim |
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127 | * |
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128 | * Authors: |
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129 | * ======== |
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130 | * |
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131 | *> \author Univ. of Tennessee |
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132 | *> \author Univ. of California Berkeley |
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133 | *> \author Univ. of Colorado Denver |
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134 | *> \author NAG Ltd. |
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135 | * |
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136 | *> \date November 2011 |
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137 | * |
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138 | *> \ingroup realOTHERcomputational |
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139 | * |
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140 | *> \par Contributors: |
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141 | * ================== |
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142 | *> |
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143 | *> A. Petitet, Computer Science Dept., Univ. of Tenn., Knoxville, USA |
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144 | * |
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145 | *> \par Further Details: |
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146 | * ===================== |
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147 | *> |
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148 | *> \verbatim |
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149 | *> |
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150 | *> The shape of the matrix V and the storage of the vectors which define |
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151 | *> the H(i) is best illustrated by the following example with n = 5 and |
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152 | *> k = 3. The elements equal to 1 are not stored; the corresponding |
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153 | *> array elements are modified but restored on exit. The rest of the |
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154 | *> array is not used. |
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155 | *> |
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156 | *> DIRECT = 'F' and STOREV = 'C': DIRECT = 'F' and STOREV = 'R': |
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157 | *> |
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158 | *> ______V_____ |
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159 | *> ( v1 v2 v3 ) / \ |
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160 | *> ( v1 v2 v3 ) ( v1 v1 v1 v1 v1 . . . . 1 ) |
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161 | *> V = ( v1 v2 v3 ) ( v2 v2 v2 v2 v2 . . . 1 ) |
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162 | *> ( v1 v2 v3 ) ( v3 v3 v3 v3 v3 . . 1 ) |
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163 | *> ( v1 v2 v3 ) |
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164 | *> . . . |
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165 | *> . . . |
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166 | *> 1 . . |
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167 | *> 1 . |
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168 | *> 1 |
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169 | *> |
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170 | *> DIRECT = 'B' and STOREV = 'C': DIRECT = 'B' and STOREV = 'R': |
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171 | *> |
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172 | *> ______V_____ |
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173 | *> 1 / \ |
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174 | *> . 1 ( 1 . . . . v1 v1 v1 v1 v1 ) |
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175 | *> . . 1 ( . 1 . . . v2 v2 v2 v2 v2 ) |
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176 | *> . . . ( . . 1 . . v3 v3 v3 v3 v3 ) |
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177 | *> . . . |
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178 | *> ( v1 v2 v3 ) |
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179 | *> ( v1 v2 v3 ) |
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180 | *> V = ( v1 v2 v3 ) |
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181 | *> ( v1 v2 v3 ) |
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182 | *> ( v1 v2 v3 ) |
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183 | *> \endverbatim |
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184 | *> |
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185 | * ===================================================================== |
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186 | SUBROUTINE SLARZT( DIRECT, STOREV, N, K, V, LDV, TAU, T, LDT ) |
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187 | * |
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188 | * -- LAPACK computational routine (version 3.4.0) -- |
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189 | * -- LAPACK is a software package provided by Univ. of Tennessee, -- |
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190 | * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..-- |
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191 | * November 2011 |
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192 | * |
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193 | * .. Scalar Arguments .. |
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194 | CHARACTER DIRECT, STOREV |
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195 | INTEGER K, LDT, LDV, N |
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196 | * .. |
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197 | * .. Array Arguments .. |
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198 | REAL T( LDT, * ), TAU( * ), V( LDV, * ) |
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199 | * .. |
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200 | * |
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201 | * ===================================================================== |
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202 | * |
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203 | * .. Parameters .. |
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204 | REAL ZERO |
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205 | PARAMETER ( ZERO = 0.0E+0 ) |
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206 | * .. |
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207 | * .. Local Scalars .. |
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208 | INTEGER I, INFO, J |
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209 | * .. |
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210 | * .. External Subroutines .. |
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211 | EXTERNAL SGEMV, STRMV, XERBLA |
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212 | * .. |
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213 | * .. External Functions .. |
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214 | LOGICAL LSAME |
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215 | EXTERNAL LSAME |
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216 | * .. |
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217 | * .. Executable Statements .. |
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218 | * |
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219 | * Check for currently supported options |
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220 | * |
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221 | INFO = 0 |
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222 | IF( .NOT.LSAME( DIRECT, 'B' ) ) THEN |
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223 | INFO = -1 |
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224 | ELSE IF( .NOT.LSAME( STOREV, 'R' ) ) THEN |
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225 | INFO = -2 |
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226 | END IF |
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227 | IF( INFO.NE.0 ) THEN |
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228 | CALL XERBLA( 'SLARZT', -INFO ) |
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229 | RETURN |
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230 | END IF |
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231 | * |
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232 | DO 20 I = K, 1, -1 |
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233 | IF( TAU( I ).EQ.ZERO ) THEN |
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234 | * |
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235 | * H(i) = I |
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236 | * |
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237 | DO 10 J = I, K |
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238 | T( J, I ) = ZERO |
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239 | 10 CONTINUE |
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240 | ELSE |
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241 | * |
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242 | * general case |
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243 | * |
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244 | IF( I.LT.K ) THEN |
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245 | * |
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246 | * T(i+1:k,i) = - tau(i) * V(i+1:k,1:n) * V(i,1:n)**T |
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247 | * |
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248 | CALL SGEMV( 'No transpose', K-I, N, -TAU( I ), |
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249 | $ V( I+1, 1 ), LDV, V( I, 1 ), LDV, ZERO, |
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250 | $ T( I+1, I ), 1 ) |
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251 | * |
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252 | * T(i+1:k,i) = T(i+1:k,i+1:k) * T(i+1:k,i) |
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253 | * |
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254 | CALL STRMV( 'Lower', 'No transpose', 'Non-unit', K-I, |
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255 | $ T( I+1, I+1 ), LDT, T( I+1, I ), 1 ) |
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256 | END IF |
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257 | T( I, I ) = TAU( I ) |
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258 | END IF |
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259 | 20 CONTINUE |
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260 | RETURN |
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261 | * |
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262 | * End of SLARZT |
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263 | * |
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264 | END |
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