1 | /* gamma.c |
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2 | * |
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3 | * Gamma function |
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4 | * |
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5 | * |
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6 | * |
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7 | * SYNOPSIS: |
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8 | * |
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9 | * double x, y, gamma(); |
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10 | * extern int sgngam; |
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11 | * |
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12 | * y = gamma( x ); |
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13 | * |
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14 | * |
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15 | * |
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16 | * DESCRIPTION: |
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17 | * |
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18 | * Returns gamma function of the argument. The result is |
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19 | * correctly signed, and the sign (+1 or -1) is also |
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20 | * returned in a global (extern) variable named sgngam. |
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21 | * This variable is also filled in by the logarithmic gamma |
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22 | * function lgam(). |
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23 | * |
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24 | * Arguments |x| <= 34 are reduced by recurrence and the function |
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25 | * approximated by a rational function of degree 6/7 in the |
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26 | * interval (2,3). Large arguments are handled by Stirling's |
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27 | * formula. Large negative arguments are made positive using |
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28 | * a reflection formula. |
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29 | * |
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30 | * |
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31 | * ACCURACY: |
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32 | * |
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33 | * Relative error: |
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34 | * arithmetic domain # trials peak rms |
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35 | * DEC -34, 34 10000 1.3e-16 2.5e-17 |
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36 | * IEEE -170,-33 20000 2.3e-15 3.3e-16 |
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37 | * IEEE -33, 33 20000 9.4e-16 2.2e-16 |
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38 | * IEEE 33, 171.6 20000 2.3e-15 3.2e-16 |
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39 | * |
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40 | * Error for arguments outside the test range will be larger |
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41 | * owing to error amplification by the exponential function. |
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42 | * |
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43 | */ |
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44 | /* lgam() |
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45 | * |
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46 | * Natural logarithm of gamma function |
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47 | * |
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48 | * |
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49 | * |
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50 | * SYNOPSIS: |
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51 | * |
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52 | * double x, y, lgam(); |
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53 | * extern int sgngam; |
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54 | * |
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55 | * y = lgam( x ); |
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56 | * |
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57 | * |
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58 | * |
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59 | * DESCRIPTION: |
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60 | * |
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61 | * Returns the base e (2.718...) logarithm of the absolute |
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62 | * value of the gamma function of the argument. |
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63 | * The sign (+1 or -1) of the gamma function is returned in a |
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64 | * global (extern) variable named sgngam. |
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65 | * |
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66 | * For arguments greater than 13, the logarithm of the gamma |
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67 | * function is approximated by the logarithmic version of |
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68 | * Stirling's formula using a polynomial approximation of |
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69 | * degree 4. Arguments between -33 and +33 are reduced by |
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70 | * recurrence to the interval [2,3] of a rational approximation. |
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71 | * The cosecant reflection formula is employed for arguments |
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72 | * less than -33. |
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73 | * |
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74 | * Arguments greater than MAXLGM return MAXNUM and an error |
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75 | * message. MAXLGM = 2.035093e36 for DEC |
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76 | * arithmetic or 2.556348e305 for IEEE arithmetic. |
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77 | * |
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78 | * |
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79 | * |
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80 | * ACCURACY: |
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81 | * |
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82 | * |
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83 | * arithmetic domain # trials peak rms |
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84 | * DEC 0, 3 7000 5.2e-17 1.3e-17 |
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85 | * DEC 2.718, 2.035e36 5000 3.9e-17 9.9e-18 |
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86 | * IEEE 0, 3 28000 5.4e-16 1.1e-16 |
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87 | * IEEE 2.718, 2.556e305 40000 3.5e-16 8.3e-17 |
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88 | * The error criterion was relative when the function magnitude |
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89 | * was greater than one but absolute when it was less than one. |
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90 | * |
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91 | * The following test used the relative error criterion, though |
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92 | * at certain points the relative error could be much higher than |
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93 | * indicated. |
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94 | * IEEE -200, -4 10000 4.8e-16 1.3e-16 |
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95 | * |
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96 | */ |
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97 | |
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98 | /* gamma.c */ |
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99 | /* gamma function */ |
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100 | |
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101 | /* |
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102 | Cephes Math Library Release 2.8: June, 2000 |
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103 | Copyright 1984, 1987, 1989, 1992, 2000 by Stephen L. Moshier |
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104 | */ |
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105 | |
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106 | |
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107 | double lanczos_gamma( double ); |
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108 | |
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109 | double lanczos_gamma( double x) { |
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110 | |
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111 | #if FLOAT_SIZE > 4 |
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112 | double p, q, u, w, z; |
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113 | int i; |
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114 | int sgngam = 1; |
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115 | |
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116 | const double LS2PI = 0.91893853320467274178; |
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117 | const double MAXLGM = 2.556348e305; |
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118 | const double MAXNUM = 1.79769313486231570815E308; |
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119 | const double LOGPI = 1.14472988584940017414; |
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120 | const double PI = M_PI; |
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121 | |
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122 | double A[8] = { |
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123 | 8.11614167470508450300E-4, |
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124 | -5.95061904284301438324E-4, |
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125 | 7.93650340457716943945E-4, |
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126 | -2.77777777730099687205E-3, |
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127 | 8.33333333333331927722E-2, |
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128 | 0.0, |
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129 | 0.0, |
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130 | 0.0 |
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131 | }; |
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132 | double B[8] = { |
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133 | -1.37825152569120859100E3, |
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134 | -3.88016315134637840924E4, |
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135 | -3.31612992738871184744E5, |
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136 | -1.16237097492762307383E6, |
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137 | -1.72173700820839662146E6, |
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138 | -8.53555664245765465627E5, |
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139 | 0.0, |
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140 | 0.0 |
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141 | }; |
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142 | |
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143 | double C[8] = { |
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144 | /* 1.00000000000000000000E0, */ |
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145 | -3.51815701436523470549E2, |
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146 | -1.70642106651881159223E4, |
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147 | -2.20528590553854454839E5, |
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148 | -1.13933444367982507207E6, |
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149 | -2.53252307177582951285E6, |
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150 | -2.01889141433532773231E6, |
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151 | 0.0, |
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152 | 0.0 |
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153 | }; |
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154 | |
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155 | sgngam = 1; |
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156 | |
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157 | if( x < -34.0 ) { |
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158 | q = -x; |
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159 | w = lanczos_gamma(q); /* note this modifies sgngam! */ |
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160 | p = floor(q); |
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161 | if( p == q ) { |
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162 | lgsing: |
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163 | goto loverf; |
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164 | } |
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165 | i = p; |
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166 | if( (i & 1) == 0 ) |
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167 | sgngam = -1; |
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168 | else |
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169 | sgngam = 1; |
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170 | z = q - p; |
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171 | if( z > 0.5 ) { |
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172 | p += 1.0; |
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173 | z = p - q; |
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174 | } |
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175 | z = q * sin( PI * z ); |
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176 | if( z == 0.0 ) |
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177 | goto lgsing; |
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178 | z = LOGPI - log( z ) - w; |
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179 | return( z ); |
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180 | } |
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181 | |
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182 | if( x < 13.0 ) { |
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183 | z = 1.0; |
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184 | p = 0.0; |
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185 | u = x; |
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186 | while( u >= 3.0 ) { |
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187 | p -= 1.0; |
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188 | u = x + p; |
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189 | z *= u; |
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190 | } |
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191 | while( u < 2.0 ) { |
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192 | if( u == 0.0 ) |
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193 | goto lgsing; |
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194 | z /= u; |
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195 | p += 1.0; |
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196 | u = x + p; |
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197 | } |
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198 | if( z < 0.0 ) { |
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199 | sgngam = -1; |
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200 | z = -z; |
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201 | } |
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202 | else |
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203 | sgngam = 1; |
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204 | if( u == 2.0 ) |
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205 | return( log(z) ); |
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206 | p -= 2.0; |
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207 | x = x + p; |
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208 | p = x * polevl( x, B, 5 ) / p1evl( x, C, 6); |
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209 | return( log(z) + p ); |
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210 | } |
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211 | |
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212 | if( x > MAXLGM ) { |
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213 | loverf: |
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214 | return( sgngam * MAXNUM ); |
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215 | } |
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216 | |
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217 | q = ( x - 0.5 ) * log(x) - x + LS2PI; |
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218 | if( x > 1.0e8 ) |
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219 | return( q ); |
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220 | |
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221 | p = 1.0/(x*x); |
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222 | if( x >= 1000.0 ) |
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223 | q += (( 7.9365079365079365079365e-4 * p |
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224 | - 2.7777777777777777777778e-3) *p |
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225 | + 0.0833333333333333333333) / x; |
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226 | else |
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227 | q += polevl( p, A, 4 ) / x; |
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228 | return( q ); |
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229 | #else |
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230 | |
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231 | double p, q, w, z, xx; |
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232 | double nx, tx; |
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233 | int i, direction; |
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234 | int sgngamf = 1; |
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235 | |
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236 | double B[8] = { |
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237 | 6.055172732649237E-004, |
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238 | -1.311620815545743E-003, |
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239 | 2.863437556468661E-003, |
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240 | -7.366775108654962E-003, |
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241 | 2.058355474821512E-002, |
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242 | -6.735323259371034E-002, |
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243 | 3.224669577325661E-001, |
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244 | 4.227843421859038E-001 |
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245 | }; |
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246 | |
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247 | /* log gamma(x+1), -.25 < x < .25 */ |
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248 | double C[8] = { |
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249 | 1.369488127325832E-001, |
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250 | -1.590086327657347E-001, |
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251 | 1.692415923504637E-001, |
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252 | -2.067882815621965E-001, |
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253 | 2.705806208275915E-001, |
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254 | -4.006931650563372E-001, |
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255 | 8.224670749082976E-001, |
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256 | -5.772156501719101E-001 |
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257 | }; |
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258 | |
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259 | /* log( sqrt( 2*pi ) ) */ |
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260 | const double LS2PI = 0.91893853320467274178; |
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261 | const double MAXLGM = 2.035093e36; |
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262 | const double PIINV = 0.318309886183790671538; |
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263 | const double MAXNUMF = 3.4028234663852885981170418348451692544e38; |
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264 | const double PIF = 3.141592653589793238; |
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265 | /* Logarithm of gamma function */ |
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266 | |
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267 | sgngamf = 1; |
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268 | |
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269 | xx = x; |
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270 | if( xx < 0.0 ) { |
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271 | q = -xx; |
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272 | w = lgamma(q); /* note this modifies sgngam! */ |
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273 | p = floor(q); |
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274 | if( p == q ) |
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275 | goto loverf; |
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276 | i = p; |
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277 | |
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278 | if( (i & 1) == 0 ) |
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279 | sgngamf = -1; |
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280 | else |
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281 | sgngamf = 1; |
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282 | |
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283 | z = q - p; |
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284 | |
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285 | if( z > 0.5 ) { |
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286 | p += 1.0; |
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287 | z = p - q; |
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288 | } |
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289 | z = q * sin( PIF * z ); |
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290 | if( z == 0.0 ) |
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291 | goto loverf; |
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292 | z = -log( PIINV*z ) - w; |
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293 | return( z ); |
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294 | } |
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295 | |
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296 | if( x < 6.5 ) { |
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297 | direction = 0; |
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298 | z = 1.0; |
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299 | tx = x; |
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300 | nx = 0.0; |
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301 | if( x >= 1.5 ) { |
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302 | while( tx > 2.5 ) { |
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303 | nx -= 1.0; |
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304 | tx = x + nx; |
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305 | z *=tx; |
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306 | } |
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307 | x += nx - 2.0; |
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308 | iv1r5: |
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309 | p = x * polevl( x, B, 7 ); |
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310 | goto cont; |
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311 | } |
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312 | if( x >= 1.25 ) { |
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313 | z *= x; |
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314 | x -= 1.0; /* x + 1 - 2 */ |
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315 | direction = 1; |
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316 | goto iv1r5; |
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317 | } |
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318 | if( x >= 0.75 ) { |
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319 | x -= 1.0; |
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320 | p = x * polevl( x, C, 7 ); |
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321 | q = 0.0; |
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322 | goto contz; |
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323 | } |
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324 | while( tx < 1.5 ) { |
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325 | if( tx == 0.0 ) |
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326 | goto loverf; |
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327 | z *=tx; |
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328 | nx += 1.0; |
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329 | tx = x + nx; |
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330 | } |
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331 | direction = 1; |
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332 | x += nx - 2.0; |
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333 | p = x * polevl( x, B, 7 ); |
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334 | |
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335 | cont: |
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336 | if( z < 0.0 ) { |
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337 | sgngamf = -1; |
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338 | z = -z; |
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339 | } |
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340 | else { |
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341 | sgngamf = 1; |
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342 | } |
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343 | q = log(z); |
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344 | if( direction ) |
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345 | q = -q; |
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346 | contz: |
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347 | return( p + q ); |
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348 | } |
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349 | |
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350 | if( x > MAXLGM ) { |
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351 | loverf: |
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352 | return( sgngamf * MAXNUMF ); |
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353 | } |
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354 | |
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355 | /* Note, though an asymptotic formula could be used for x >= 3, |
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356 | * there is cancellation error in the following if x < 6.5. */ |
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357 | q = LS2PI - x; |
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358 | q += ( x - 0.5 ) * log(x); |
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359 | |
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360 | if( x <= 1.0e4 ) { |
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361 | z = 1.0/x; |
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362 | p = z * z; |
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363 | q += (( 6.789774945028216E-004 * p |
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364 | - 2.769887652139868E-003 ) * p |
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365 | + 8.333316229807355E-002 ) * z; |
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366 | } |
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367 | return( q ); |
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368 | #endif |
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369 | } |
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