1 | /* jn.c |
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2 | * |
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3 | * Bessel function of integer order |
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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 | * int n; |
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10 | * double x, y, jn(); |
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11 | * |
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12 | * y = jn( n, 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 Bessel function of order n, where n is a |
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19 | * (possibly negative) integer. |
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20 | * |
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21 | * The ratio of jn(x) to j0(x) is computed by backward |
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22 | * recurrence. First the ratio jn/jn-1 is found by a |
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23 | * continued fraction expansion. Then the recurrence |
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24 | * relating successive orders is applied until j0 or j1 is |
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25 | * reached. |
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26 | * |
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27 | * If n = 0 or 1 the routine for j0 or j1 is called |
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28 | * directly. |
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29 | * |
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30 | * |
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31 | * |
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32 | * ACCURACY: |
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33 | * |
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34 | * Absolute error: |
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35 | * arithmetic range # trials peak rms |
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36 | * DEC 0, 30 5500 6.9e-17 9.3e-18 |
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37 | * IEEE 0, 30 5000 4.4e-16 7.9e-17 |
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38 | * |
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39 | * |
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40 | * Not suitable for large n or x. Use jv() instead. |
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41 | * |
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42 | */ |
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43 | |
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44 | /* jn.c |
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45 | Cephes Math Library Release 2.8: June, 2000 |
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46 | Copyright 1984, 1987, 2000 by Stephen L. Moshier |
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47 | */ |
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48 | |
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49 | double sas_JN( int n, double x ); |
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50 | |
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51 | double sas_JN( int n, double x ) { |
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52 | |
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53 | const double MACHEP = 1.11022302462515654042E-16; |
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54 | double pkm2, pkm1, pk, xk, r, ans; |
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55 | int k, sign; |
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56 | |
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57 | if( n < 0 ) { |
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58 | n = -n; |
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59 | if( (n & 1) == 0 ) /* -1**n */ |
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60 | sign = 1; |
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61 | else |
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62 | sign = -1; |
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63 | } |
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64 | else |
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65 | sign = 1; |
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66 | |
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67 | if( x < 0.0 ) { |
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68 | if( n & 1 ) |
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69 | sign = -sign; |
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70 | x = -x; |
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71 | } |
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72 | |
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73 | if( n == 0 ) |
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74 | return( sign * sas_J0(x) ); |
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75 | if( n == 1 ) |
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76 | return( sign * sas_J1(x) ); |
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77 | if( n == 2 ) |
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78 | return( sign * (2.0 * sas_J1(x) / x - sas_J0(x)) ); |
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79 | |
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80 | if( x < MACHEP ) |
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81 | return( 0.0 ); |
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82 | |
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83 | #if FLOAT_SIZE > 4 |
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84 | k = 53; |
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85 | #else |
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86 | k = 24; |
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87 | #endif |
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88 | |
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89 | pk = 2 * (n + k); |
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90 | ans = pk; |
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91 | xk = x * x; |
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92 | |
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93 | do { |
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94 | pk -= 2.0; |
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95 | ans = pk - (xk/ans); |
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96 | } while( --k > 0 ); |
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97 | |
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98 | /* backward recurrence */ |
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99 | |
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100 | pk = 1.0; |
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101 | |
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102 | #if FLOAT_SIZE > 4 |
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103 | ans = x/ans; |
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104 | pkm1 = 1.0/ans; |
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105 | |
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106 | k = n-1; |
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107 | r = 2 * k; |
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108 | |
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109 | do { |
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110 | pkm2 = (pkm1 * r - pk * x) / x; |
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111 | pk = pkm1; |
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112 | pkm1 = pkm2; |
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113 | r -= 2.0; |
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114 | } while( --k > 0 ); |
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115 | |
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116 | if( fabs(pk) > fabs(pkm1) ) |
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117 | ans = sas_J1(x)/pk; |
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118 | else |
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119 | ans = sas_J0(x)/pkm1; |
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120 | |
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121 | return( sign * ans ); |
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122 | |
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123 | #else |
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124 | const double xinv = 1.0/x; |
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125 | pkm1 = ans * xinv; |
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126 | k = n-1; |
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127 | r = (float )(2 * k); |
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128 | |
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129 | do { |
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130 | pkm2 = (pkm1 * r - pk * x) * xinv; |
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131 | pk = pkm1; |
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132 | pkm1 = pkm2; |
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133 | r -= 2.0; |
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134 | } |
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135 | while( --k > 0 ); |
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136 | |
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137 | r = pk; |
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138 | if( r < 0 ) |
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139 | r = -r; |
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140 | ans = pkm1; |
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141 | if( ans < 0 ) |
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142 | ans = -ans; |
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143 | |
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144 | if( r > ans ) /* if( fabs(pk) > fabs(pkm1) ) */ |
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145 | ans = sign * sas_J1(x)/pk; |
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146 | else |
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147 | ans = sign * sas_J0(x)/pkm1; |
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148 | return( ans ); |
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149 | #endif |
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150 | } |
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151 | |
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