1 | /* j1.c |
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2 | * |
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3 | * Bessel function of order one |
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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, j1(); |
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10 | * |
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11 | * y = j1( x ); |
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12 | * |
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13 | * |
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14 | * |
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15 | * DESCRIPTION: |
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16 | * |
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17 | * Returns Bessel function of order one of the argument. |
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18 | * |
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19 | * The domain is divided into the intervals [0, 8] and |
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20 | * (8, infinity). In the first interval a 24 term Chebyshev |
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21 | * expansion is used. In the second, the asymptotic |
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22 | * trigonometric representation is employed using two |
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23 | * rational functions of degree 5/5. |
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24 | * |
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25 | * |
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26 | * |
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27 | * ACCURACY: |
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28 | * |
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29 | * Absolute error: |
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30 | * arithmetic domain # trials peak rms |
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31 | * DEC 0, 30 10000 4.0e-17 1.1e-17 |
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32 | * IEEE 0, 30 30000 2.6e-16 1.1e-16 |
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33 | * |
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34 | * |
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35 | */ |
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36 | |
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37 | /* |
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38 | Cephes Math Library Release 2.8: June, 2000 |
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39 | Copyright 1984, 1987, 1989, 2000 by Stephen L. Moshier |
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40 | */ |
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41 | |
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42 | double J1(double x) { |
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43 | |
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44 | //Cephes double pression function |
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45 | #if FLOAT_SIZE>4 |
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46 | |
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47 | double w, z, p, q, xn; |
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48 | |
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49 | const double Z1 = 1.46819706421238932572E1; |
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50 | const double Z2 = 4.92184563216946036703E1; |
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51 | const double THPIO4 = 2.35619449019234492885; |
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52 | const double SQ2OPI = 0.79788456080286535588; |
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53 | |
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54 | w = x; |
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55 | if( x < 0 ) |
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56 | w = -x; |
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57 | |
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58 | if( w <= 5.0 ) |
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59 | { |
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60 | z = x * x; |
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61 | w = polevl( z, RPJ1, 3 ) / p1evl( z, RQJ1, 8 ); |
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62 | w = w * x * (z - Z1) * (z - Z2); |
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63 | return( w ); |
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64 | } |
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65 | |
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66 | w = 5.0/x; |
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67 | z = w * w; |
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68 | |
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69 | p = polevl( z, PPJ1, 6)/polevl( z, PQJ1, 6 ); |
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70 | q = polevl( z, QPJ1, 7)/p1evl( z, QQJ1, 7 ); |
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71 | |
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72 | xn = x - THPIO4; |
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73 | |
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74 | double sn, cn; |
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75 | SINCOS(xn, sn, cn); |
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76 | p = p * cn - w * q * sn; |
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77 | |
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78 | return( p * SQ2OPI / sqrt(x) ); |
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79 | |
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80 | |
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81 | //Single precission version of cephes |
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82 | #else |
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83 | double xx, w, z, p, q, xn; |
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84 | |
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85 | const double Z1 = 1.46819706421238932572E1; |
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86 | const double THPIO4F = 2.35619449019234492885; /* 3*pi/4 */ |
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87 | |
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88 | |
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89 | xx = x; |
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90 | if( xx < 0 ) |
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91 | xx = -x; |
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92 | |
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93 | if( xx <= 2.0 ) |
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94 | { |
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95 | z = xx * xx; |
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96 | p = (z-Z1) * xx * polevl( z, JPJ1, 4 ); |
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97 | return( p ); |
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98 | } |
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99 | |
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100 | q = 1.0/x; |
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101 | w = sqrt(q); |
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102 | |
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103 | p = w * polevl( q, MO1J1, 7); |
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104 | w = q*q; |
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105 | xn = q * polevl( w, PH1J1, 7) - THPIO4F; |
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106 | p = p * cos(xn + xx); |
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107 | |
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108 | return(p); |
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109 | #endif |
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110 | } |
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111 | |
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112 | //Finally J1c function that equals 2*J1(x)/x |
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113 | double J1c(double x) { |
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114 | return (x != 0.0 ) ? 2.0*J1(x)/x : 1.0; |
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115 | } |
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