1 /* @(#)e_jn.c 5.1 93/09/24 */ 2 /* 3 * ==================================================== 4 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved. 5 * 6 * Developed at SunPro, a Sun Microsystems, Inc. business. 7 * Permission to use, copy, modify, and distribute this 8 * software is freely granted, provided that this notice 9 * is preserved. 10 * ==================================================== 11 */ 12 13 #ifndef lint 14 static char rcsid[] = "$Id: e_jn.c,v 1.4 1994/03/03 17:04:16 jtc Exp $"; 15 #endif 16 17 /* 18 * __ieee754_jn(n, x), __ieee754_yn(n, x) 19 * floating point Bessel's function of the 1st and 2nd kind 20 * of order n 21 * 22 * Special cases: 23 * y0(0)=y1(0)=yn(n,0) = -inf with division by zero signal; 24 * y0(-ve)=y1(-ve)=yn(n,-ve) are NaN with invalid signal. 25 * Note 2. About jn(n,x), yn(n,x) 26 * For n=0, j0(x) is called, 27 * for n=1, j1(x) is called, 28 * for n<x, forward recursion us used starting 29 * from values of j0(x) and j1(x). 30 * for n>x, a continued fraction approximation to 31 * j(n,x)/j(n-1,x) is evaluated and then backward 32 * recursion is used starting from a supposed value 33 * for j(n,x). The resulting value of j(0,x) is 34 * compared with the actual value to correct the 35 * supposed value of j(n,x). 36 * 37 * yn(n,x) is similar in all respects, except 38 * that forward recursion is used for all 39 * values of n>1. 40 * 41 */ 42 43 #include <math.h> 44 #include <machine/endian.h> 45 46 #if BYTE_ORDER == LITTLE_ENDIAN 47 #define n0 1 48 #else 49 #define n0 0 50 #endif 51 52 #ifdef __STDC__ 53 static const double 54 #else 55 static double 56 #endif 57 invsqrtpi= 5.64189583547756279280e-01, /* 0x3FE20DD7, 0x50429B6D */ 58 two = 2.00000000000000000000e+00, /* 0x40000000, 0x00000000 */ 59 one = 1.00000000000000000000e+00; /* 0x3FF00000, 0x00000000 */ 60 61 static double zero = 0.00000000000000000000e+00; 62 63 #ifdef __STDC__ 64 double __ieee754_jn(int n, double x) 65 #else 66 double __ieee754_jn(n,x) 67 int n; double x; 68 #endif 69 { 70 int i,hx,ix,lx, sgn; 71 double a, b, temp, di; 72 double z, w; 73 74 /* J(-n,x) = (-1)^n * J(n, x), J(n, -x) = (-1)^n * J(n, x) 75 * Thus, J(-n,x) = J(n,-x) 76 */ 77 hx = *(n0+(int*)&x); 78 ix = 0x7fffffff&hx; 79 lx = *(1-n0+(int*)&x); 80 /* if J(n,NaN) is NaN */ 81 if((ix|((unsigned)(lx|-lx))>>31)>0x7ff00000) return x+x; 82 if(n<0){ 83 n = -n; 84 x = -x; 85 hx ^= 0x80000000; 86 } 87 if(n==0) return(__ieee754_j0(x)); 88 if(n==1) return(__ieee754_j1(x)); 89 sgn = (n&1)&(hx>>31); /* even n -- 0, odd n -- sign(x) */ 90 x = fabs(x); 91 if((ix|lx)==0||ix>=0x7ff00000) /* if x is 0 or inf */ 92 b = zero; 93 else if((double)n<=x) { 94 /* Safe to use J(n+1,x)=2n/x *J(n,x)-J(n-1,x) */ 95 if(ix>=0x52D00000) { /* x > 2**302 */ 96 /* (x >> n**2) 97 * Jn(x) = cos(x-(2n+1)*pi/4)*sqrt(2/x*pi) 98 * Yn(x) = sin(x-(2n+1)*pi/4)*sqrt(2/x*pi) 99 * Let s=sin(x), c=cos(x), 100 * xn=x-(2n+1)*pi/4, sqt2 = sqrt(2),then 101 * 102 * n sin(xn)*sqt2 cos(xn)*sqt2 103 * ---------------------------------- 104 * 0 s-c c+s 105 * 1 -s-c -c+s 106 * 2 -s+c -c-s 107 * 3 s+c c-s 108 */ 109 switch(n&3) { 110 case 0: temp = cos(x)+sin(x); break; 111 case 1: temp = -cos(x)+sin(x); break; 112 case 2: temp = -cos(x)-sin(x); break; 113 case 3: temp = cos(x)-sin(x); break; 114 } 115 b = invsqrtpi*temp/sqrt(x); 116 } else { 117 a = __ieee754_j0(x); 118 b = __ieee754_j1(x); 119 for(i=1;i<n;i++){ 120 temp = b; 121 b = b*((double)(i+i)/x) - a; /* avoid underflow */ 122 a = temp; 123 } 124 } 125 } else { 126 if(ix<0x3e100000) { /* x < 2**-29 */ 127 /* x is tiny, return the first Taylor expansion of J(n,x) 128 * J(n,x) = 1/n!*(x/2)^n - ... 129 */ 130 if(n>33) /* underflow */ 131 b = zero; 132 else { 133 temp = x*0.5; b = temp; 134 for (a=one,i=2;i<=n;i++) { 135 a *= (double)i; /* a = n! */ 136 b *= temp; /* b = (x/2)^n */ 137 } 138 b = b/a; 139 } 140 } else { 141 /* use backward recurrence */ 142 /* x x^2 x^2 143 * J(n,x)/J(n-1,x) = ---- ------ ------ ..... 144 * 2n - 2(n+1) - 2(n+2) 145 * 146 * 1 1 1 147 * (for large x) = ---- ------ ------ ..... 148 * 2n 2(n+1) 2(n+2) 149 * -- - ------ - ------ - 150 * x x x 151 * 152 * Let w = 2n/x and h=2/x, then the above quotient 153 * is equal to the continued fraction: 154 * 1 155 * = ----------------------- 156 * 1 157 * w - ----------------- 158 * 1 159 * w+h - --------- 160 * w+2h - ... 161 * 162 * To determine how many terms needed, let 163 * Q(0) = w, Q(1) = w(w+h) - 1, 164 * Q(k) = (w+k*h)*Q(k-1) - Q(k-2), 165 * When Q(k) > 1e4 good for single 166 * When Q(k) > 1e9 good for double 167 * When Q(k) > 1e17 good for quadruple 168 */ 169 /* determine k */ 170 double t,v; 171 double q0,q1,h,tmp; int k,m; 172 w = (n+n)/(double)x; h = 2.0/(double)x; 173 q0 = w; z = w+h; q1 = w*z - 1.0; k=1; 174 while(q1<1.0e9) { 175 k += 1; z += h; 176 tmp = z*q1 - q0; 177 q0 = q1; 178 q1 = tmp; 179 } 180 m = n+n; 181 for(t=zero, i = 2*(n+k); i>=m; i -= 2) t = one/(i/x-t); 182 a = t; 183 b = one; 184 /* estimate log((2/x)^n*n!) = n*log(2/x)+n*ln(n) 185 * Hence, if n*(log(2n/x)) > ... 186 * single 8.8722839355e+01 187 * double 7.09782712893383973096e+02 188 * long double 1.1356523406294143949491931077970765006170e+04 189 * then recurrent value may overflow and the result is 190 * likely underflow to zero 191 */ 192 tmp = n; 193 v = two/x; 194 tmp = tmp*__ieee754_log(fabs(v*tmp)); 195 if(tmp<7.09782712893383973096e+02) { 196 for(i=n-1,di=(double)(i+i);i>0;i--){ 197 temp = b; 198 b *= di; 199 b = b/x - a; 200 a = temp; 201 di -= two; 202 } 203 } else { 204 for(i=n-1,di=(double)(i+i);i>0;i--){ 205 temp = b; 206 b *= di; 207 b = b/x - a; 208 a = temp; 209 di -= two; 210 /* scale b to avoid spurious overflow */ 211 if(b>1e100) { 212 a /= b; 213 t /= b; 214 b = one; 215 } 216 } 217 } 218 b = (t*__ieee754_j0(x)/b); 219 } 220 } 221 if(sgn==1) return -b; else return b; 222 } 223 224 #ifdef __STDC__ 225 double __ieee754_yn(int n, double x) 226 #else 227 double __ieee754_yn(n,x) 228 int n; double x; 229 #endif 230 { 231 int i,hx,ix,lx; 232 int sign; 233 double a, b, temp; 234 235 hx = *(n0+(int*)&x); 236 ix = 0x7fffffff&hx; 237 lx = *(1-n0+(int*)&x); 238 /* if Y(n,NaN) is NaN */ 239 if((ix|((unsigned)(lx|-lx))>>31)>0x7ff00000) return x+x; 240 if((ix|lx)==0) return -one/zero; 241 if(hx<0) return zero/zero; 242 sign = 1; 243 if(n<0){ 244 n = -n; 245 sign = 1 - ((n&1)<<2); 246 } 247 if(n==0) return(__ieee754_y0(x)); 248 if(n==1) return(sign*__ieee754_y1(x)); 249 if(ix==0x7ff00000) return zero; 250 if(ix>=0x52D00000) { /* x > 2**302 */ 251 /* (x >> n**2) 252 * Jn(x) = cos(x-(2n+1)*pi/4)*sqrt(2/x*pi) 253 * Yn(x) = sin(x-(2n+1)*pi/4)*sqrt(2/x*pi) 254 * Let s=sin(x), c=cos(x), 255 * xn=x-(2n+1)*pi/4, sqt2 = sqrt(2),then 256 * 257 * n sin(xn)*sqt2 cos(xn)*sqt2 258 * ---------------------------------- 259 * 0 s-c c+s 260 * 1 -s-c -c+s 261 * 2 -s+c -c-s 262 * 3 s+c c-s 263 */ 264 switch(n&3) { 265 case 0: temp = sin(x)-cos(x); break; 266 case 1: temp = -sin(x)-cos(x); break; 267 case 2: temp = -sin(x)+cos(x); break; 268 case 3: temp = sin(x)+cos(x); break; 269 } 270 b = invsqrtpi*temp/sqrt(x); 271 } else { 272 a = __ieee754_y0(x); 273 b = __ieee754_y1(x); 274 /* quit if b is -inf */ 275 for(i=1;i<n&&(*(n0+(int*)&b)!=0xfff00000);i++){ 276 temp = b; 277 b = ((double)(i+i)/x)*b - a; 278 a = temp; 279 } 280 } 281 if(sign>0) return b; else return -b; 282 } 283