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1 747 jeremybenn
// Copyright 2010 The Go Authors. All rights reserved.
2
// Use of this source code is governed by a BSD-style
3
// license that can be found in the LICENSE file.
4
 
5
package math
6
 
7
/*
8
        Bessel function of the first and second kinds of order zero.
9
*/
10
 
11
// The original C code and the long comment below are
12
// from FreeBSD's /usr/src/lib/msun/src/e_j0.c and
13
// came with this notice.  The go code is a simplified
14
// version of the original C.
15
//
16
// ====================================================
17
// Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
18
//
19
// Developed at SunPro, a Sun Microsystems, Inc. business.
20
// Permission to use, copy, modify, and distribute this
21
// software is freely granted, provided that this notice
22
// is preserved.
23
// ====================================================
24
//
25
// __ieee754_j0(x), __ieee754_y0(x)
26
// Bessel function of the first and second kinds of order zero.
27
// Method -- j0(x):
28
//      1. For tiny x, we use j0(x) = 1 - x**2/4 + x**4/64 - ...
29
//      2. Reduce x to |x| since j0(x)=j0(-x),  and
30
//         for x in (0,2)
31
//              j0(x) = 1-z/4+ z**2*R0/S0,  where z = x*x;
32
//         (precision:  |j0-1+z/4-z**2R0/S0 |<2**-63.67 )
33
//         for x in (2,inf)
34
//              j0(x) = sqrt(2/(pi*x))*(p0(x)*cos(x0)-q0(x)*sin(x0))
35
//         where x0 = x-pi/4. It is better to compute sin(x0),cos(x0)
36
//         as follow:
37
//              cos(x0) = cos(x)cos(pi/4)+sin(x)sin(pi/4)
38
//                      = 1/sqrt(2) * (cos(x) + sin(x))
39
//              sin(x0) = sin(x)cos(pi/4)-cos(x)sin(pi/4)
40
//                      = 1/sqrt(2) * (sin(x) - cos(x))
41
//         (To avoid cancellation, use
42
//              sin(x) +- cos(x) = -cos(2x)/(sin(x) -+ cos(x))
43
//         to compute the worse one.)
44
//
45
//      3 Special cases
46
//              j0(nan)= nan
47
//              j0(0) = 1
48
//              j0(inf) = 0
49
//
50
// Method -- y0(x):
51
//      1. For x<2.
52
//         Since
53
//              y0(x) = 2/pi*(j0(x)*(ln(x/2)+Euler) + x**2/4 - ...)
54
//         therefore y0(x)-2/pi*j0(x)*ln(x) is an even function.
55
//         We use the following function to approximate y0,
56
//              y0(x) = U(z)/V(z) + (2/pi)*(j0(x)*ln(x)), z= x**2
57
//         where
58
//              U(z) = u00 + u01*z + ... + u06*z**6
59
//              V(z) = 1  + v01*z + ... + v04*z**4
60
//         with absolute approximation error bounded by 2**-72.
61
//         Note: For tiny x, U/V = u0 and j0(x)~1, hence
62
//              y0(tiny) = u0 + (2/pi)*ln(tiny), (choose tiny<2**-27)
63
//      2. For x>=2.
64
//              y0(x) = sqrt(2/(pi*x))*(p0(x)*cos(x0)+q0(x)*sin(x0))
65
//         where x0 = x-pi/4. It is better to compute sin(x0),cos(x0)
66
//         by the method mentioned above.
67
//      3. Special cases: y0(0)=-inf, y0(x<0)=NaN, y0(inf)=0.
68
//
69
 
70
// J0 returns the order-zero Bessel function of the first kind.
71
//
72
// Special cases are:
73
//      J0(±Inf) = 0
74
//      J0(0) = 1
75
//      J0(NaN) = NaN
76
func J0(x float64) float64 {
77
        const (
78
                Huge   = 1e300
79
                TwoM27 = 1.0 / (1 << 27) // 2**-27 0x3e40000000000000
80
                TwoM13 = 1.0 / (1 << 13) // 2**-13 0x3f20000000000000
81
                Two129 = 1 << 129        // 2**129 0x4800000000000000
82
                // R0/S0 on [0, 2]
83
                R02 = 1.56249999999999947958e-02  // 0x3F8FFFFFFFFFFFFD
84
                R03 = -1.89979294238854721751e-04 // 0xBF28E6A5B61AC6E9
85
                R04 = 1.82954049532700665670e-06  // 0x3EBEB1D10C503919
86
                R05 = -4.61832688532103189199e-09 // 0xBE33D5E773D63FCE
87
                S01 = 1.56191029464890010492e-02  // 0x3F8FFCE882C8C2A4
88
                S02 = 1.16926784663337450260e-04  // 0x3F1EA6D2DD57DBF4
89
                S03 = 5.13546550207318111446e-07  // 0x3EA13B54CE84D5A9
90
                S04 = 1.16614003333790000205e-09  // 0x3E1408BCF4745D8F
91
        )
92
        // special cases
93
        switch {
94
        case IsNaN(x):
95
                return x
96
        case IsInf(x, 0):
97
                return 0
98
        case x == 0:
99
                return 1
100
        }
101
 
102
        if x < 0 {
103
                x = -x
104
        }
105
        if x >= 2 {
106
                s, c := Sincos(x)
107
                ss := s - c
108
                cc := s + c
109
 
110
                // make sure x+x does not overflow
111
                if x < MaxFloat64/2 {
112
                        z := -Cos(x + x)
113
                        if s*c < 0 {
114
                                cc = z / ss
115
                        } else {
116
                                ss = z / cc
117
                        }
118
                }
119
 
120
                // j0(x) = 1/sqrt(pi) * (P(0,x)*cc - Q(0,x)*ss) / sqrt(x)
121
                // y0(x) = 1/sqrt(pi) * (P(0,x)*ss + Q(0,x)*cc) / sqrt(x)
122
 
123
                var z float64
124
                if x > Two129 { // |x| > ~6.8056e+38
125
                        z = (1 / SqrtPi) * cc / Sqrt(x)
126
                } else {
127
                        u := pzero(x)
128
                        v := qzero(x)
129
                        z = (1 / SqrtPi) * (u*cc - v*ss) / Sqrt(x)
130
                }
131
                return z // |x| >= 2.0
132
        }
133
        if x < TwoM13 { // |x| < ~1.2207e-4
134
                if x < TwoM27 {
135
                        return 1 // |x| < ~7.4506e-9
136
                }
137
                return 1 - 0.25*x*x // ~7.4506e-9 < |x| < ~1.2207e-4
138
        }
139
        z := x * x
140
        r := z * (R02 + z*(R03+z*(R04+z*R05)))
141
        s := 1 + z*(S01+z*(S02+z*(S03+z*S04)))
142
        if x < 1 {
143
                return 1 + z*(-0.25+(r/s)) // |x| < 1.00
144
        }
145
        u := 0.5 * x
146
        return (1+u)*(1-u) + z*(r/s) // 1.0 < |x| < 2.0
147
}
148
 
149
// Y0 returns the order-zero Bessel function of the second kind.
150
//
151
// Special cases are:
152
//      Y0(+Inf) = 0
153
//      Y0(0) = -Inf
154
//      Y0(x < 0) = NaN
155
//      Y0(NaN) = NaN
156
func Y0(x float64) float64 {
157
        const (
158
                TwoM27 = 1.0 / (1 << 27)             // 2**-27 0x3e40000000000000
159
                Two129 = 1 << 129                    // 2**129 0x4800000000000000
160
                U00    = -7.38042951086872317523e-02 // 0xBFB2E4D699CBD01F
161
                U01    = 1.76666452509181115538e-01  // 0x3FC69D019DE9E3FC
162
                U02    = -1.38185671945596898896e-02 // 0xBF8C4CE8B16CFA97
163
                U03    = 3.47453432093683650238e-04  // 0x3F36C54D20B29B6B
164
                U04    = -3.81407053724364161125e-06 // 0xBECFFEA773D25CAD
165
                U05    = 1.95590137035022920206e-08  // 0x3E5500573B4EABD4
166
                U06    = -3.98205194132103398453e-11 // 0xBDC5E43D693FB3C8
167
                V01    = 1.27304834834123699328e-02  // 0x3F8A127091C9C71A
168
                V02    = 7.60068627350353253702e-05  // 0x3F13ECBBF578C6C1
169
                V03    = 2.59150851840457805467e-07  // 0x3E91642D7FF202FD
170
                V04    = 4.41110311332675467403e-10  // 0x3DFE50183BD6D9EF
171
        )
172
        // special cases
173
        switch {
174
        case x < 0 || IsNaN(x):
175
                return NaN()
176
        case IsInf(x, 1):
177
                return 0
178
        case x == 0:
179
                return Inf(-1)
180
        }
181
 
182
        if x >= 2 { // |x| >= 2.0
183
 
184
                // y0(x) = sqrt(2/(pi*x))*(p0(x)*sin(x0)+q0(x)*cos(x0))
185
                //     where x0 = x-pi/4
186
                // Better formula:
187
                //     cos(x0) = cos(x)cos(pi/4)+sin(x)sin(pi/4)
188
                //             =  1/sqrt(2) * (sin(x) + cos(x))
189
                //     sin(x0) = sin(x)cos(3pi/4)-cos(x)sin(3pi/4)
190
                //             =  1/sqrt(2) * (sin(x) - cos(x))
191
                // To avoid cancellation, use
192
                //     sin(x) +- cos(x) = -cos(2x)/(sin(x) -+ cos(x))
193
                // to compute the worse one.
194
 
195
                s, c := Sincos(x)
196
                ss := s - c
197
                cc := s + c
198
 
199
                // j0(x) = 1/sqrt(pi) * (P(0,x)*cc - Q(0,x)*ss) / sqrt(x)
200
                // y0(x) = 1/sqrt(pi) * (P(0,x)*ss + Q(0,x)*cc) / sqrt(x)
201
 
202
                // make sure x+x does not overflow
203
                if x < MaxFloat64/2 {
204
                        z := -Cos(x + x)
205
                        if s*c < 0 {
206
                                cc = z / ss
207
                        } else {
208
                                ss = z / cc
209
                        }
210
                }
211
                var z float64
212
                if x > Two129 { // |x| > ~6.8056e+38
213
                        z = (1 / SqrtPi) * ss / Sqrt(x)
214
                } else {
215
                        u := pzero(x)
216
                        v := qzero(x)
217
                        z = (1 / SqrtPi) * (u*ss + v*cc) / Sqrt(x)
218
                }
219
                return z // |x| >= 2.0
220
        }
221
        if x <= TwoM27 {
222
                return U00 + (2/Pi)*Log(x) // |x| < ~7.4506e-9
223
        }
224
        z := x * x
225
        u := U00 + z*(U01+z*(U02+z*(U03+z*(U04+z*(U05+z*U06)))))
226
        v := 1 + z*(V01+z*(V02+z*(V03+z*V04)))
227
        return u/v + (2/Pi)*J0(x)*Log(x) // ~7.4506e-9 < |x| < 2.0
228
}
229
 
230
// The asymptotic expansions of pzero is
231
//      1 - 9/128 s**2 + 11025/98304 s**4 - ..., where s = 1/x.
232
// For x >= 2, We approximate pzero by
233
//      pzero(x) = 1 + (R/S)
234
// where  R = pR0 + pR1*s**2 + pR2*s**4 + ... + pR5*s**10
235
//        S = 1 + pS0*s**2 + ... + pS4*s**10
236
// and
237
//      | pzero(x)-1-R/S | <= 2  ** ( -60.26)
238
 
239
// for x in [inf, 8]=1/[0,0.125]
240
var p0R8 = [6]float64{
241
        0.00000000000000000000e+00,  // 0x0000000000000000
242
        -7.03124999999900357484e-02, // 0xBFB1FFFFFFFFFD32
243
        -8.08167041275349795626e+00, // 0xC02029D0B44FA779
244
        -2.57063105679704847262e+02, // 0xC07011027B19E863
245
        -2.48521641009428822144e+03, // 0xC0A36A6ECD4DCAFC
246
        -5.25304380490729545272e+03, // 0xC0B4850B36CC643D
247
}
248
var p0S8 = [5]float64{
249
        1.16534364619668181717e+02, // 0x405D223307A96751
250
        3.83374475364121826715e+03, // 0x40ADF37D50596938
251
        4.05978572648472545552e+04, // 0x40E3D2BB6EB6B05F
252
        1.16752972564375915681e+05, // 0x40FC810F8F9FA9BD
253
        4.76277284146730962675e+04, // 0x40E741774F2C49DC
254
}
255
 
256
// for x in [8,4.5454]=1/[0.125,0.22001]
257
var p0R5 = [6]float64{
258
        -1.14125464691894502584e-11, // 0xBDA918B147E495CC
259
        -7.03124940873599280078e-02, // 0xBFB1FFFFE69AFBC6
260
        -4.15961064470587782438e+00, // 0xC010A370F90C6BBF
261
        -6.76747652265167261021e+01, // 0xC050EB2F5A7D1783
262
        -3.31231299649172967747e+02, // 0xC074B3B36742CC63
263
        -3.46433388365604912451e+02, // 0xC075A6EF28A38BD7
264
}
265
var p0S5 = [5]float64{
266
        6.07539382692300335975e+01, // 0x404E60810C98C5DE
267
        1.05125230595704579173e+03, // 0x40906D025C7E2864
268
        5.97897094333855784498e+03, // 0x40B75AF88FBE1D60
269
        9.62544514357774460223e+03, // 0x40C2CCB8FA76FA38
270
        2.40605815922939109441e+03, // 0x40A2CC1DC70BE864
271
}
272
 
273
// for x in [4.547,2.8571]=1/[0.2199,0.35001]
274
var p0R3 = [6]float64{
275
        -2.54704601771951915620e-09, // 0xBE25E1036FE1AA86
276
        -7.03119616381481654654e-02, // 0xBFB1FFF6F7C0E24B
277
        -2.40903221549529611423e+00, // 0xC00345B2AEA48074
278
        -2.19659774734883086467e+01, // 0xC035F74A4CB94E14
279
        -5.80791704701737572236e+01, // 0xC04D0A22420A1A45
280
        -3.14479470594888503854e+01, // 0xC03F72ACA892D80F
281
}
282
var p0S3 = [5]float64{
283
        3.58560338055209726349e+01, // 0x4041ED9284077DD3
284
        3.61513983050303863820e+02, // 0x40769839464A7C0E
285
        1.19360783792111533330e+03, // 0x4092A66E6D1061D6
286
        1.12799679856907414432e+03, // 0x40919FFCB8C39B7E
287
        1.73580930813335754692e+02, // 0x4065B296FC379081
288
}
289
 
290
// for x in [2.8570,2]=1/[0.3499,0.5]
291
var p0R2 = [6]float64{
292
        -8.87534333032526411254e-08, // 0xBE77D316E927026D
293
        -7.03030995483624743247e-02, // 0xBFB1FF62495E1E42
294
        -1.45073846780952986357e+00, // 0xBFF736398A24A843
295
        -7.63569613823527770791e+00, // 0xC01E8AF3EDAFA7F3
296
        -1.11931668860356747786e+01, // 0xC02662E6C5246303
297
        -3.23364579351335335033e+00, // 0xC009DE81AF8FE70F
298
}
299
var p0S2 = [5]float64{
300
        2.22202997532088808441e+01, // 0x40363865908B5959
301
        1.36206794218215208048e+02, // 0x4061069E0EE8878F
302
        2.70470278658083486789e+02, // 0x4070E78642EA079B
303
        1.53875394208320329881e+02, // 0x40633C033AB6FAFF
304
        1.46576176948256193810e+01, // 0x402D50B344391809
305
}
306
 
307
func pzero(x float64) float64 {
308
        var p [6]float64
309
        var q [5]float64
310
        if x >= 8 {
311
                p = p0R8
312
                q = p0S8
313
        } else if x >= 4.5454 {
314
                p = p0R5
315
                q = p0S5
316
        } else if x >= 2.8571 {
317
                p = p0R3
318
                q = p0S3
319
        } else if x >= 2 {
320
                p = p0R2
321
                q = p0S2
322
        }
323
        z := 1 / (x * x)
324
        r := p[0] + z*(p[1]+z*(p[2]+z*(p[3]+z*(p[4]+z*p[5]))))
325
        s := 1 + z*(q[0]+z*(q[1]+z*(q[2]+z*(q[3]+z*q[4]))))
326
        return 1 + r/s
327
}
328
 
329
// For x >= 8, the asymptotic expansions of qzero is
330
//      -1/8 s + 75/1024 s**3 - ..., where s = 1/x.
331
// We approximate pzero by
332
//      qzero(x) = s*(-1.25 + (R/S))
333
// where R = qR0 + qR1*s**2 + qR2*s**4 + ... + qR5*s**10
334
//       S = 1 + qS0*s**2 + ... + qS5*s**12
335
// and
336
//      | qzero(x)/s +1.25-R/S | <= 2**(-61.22)
337
 
338
// for x in [inf, 8]=1/[0,0.125]
339
var q0R8 = [6]float64{
340
        0.00000000000000000000e+00, // 0x0000000000000000
341
        7.32421874999935051953e-02, // 0x3FB2BFFFFFFFFE2C
342
        1.17682064682252693899e+01, // 0x402789525BB334D6
343
        5.57673380256401856059e+02, // 0x40816D6315301825
344
        8.85919720756468632317e+03, // 0x40C14D993E18F46D
345
        3.70146267776887834771e+04, // 0x40E212D40E901566
346
}
347
var q0S8 = [6]float64{
348
        1.63776026895689824414e+02,  // 0x406478D5365B39BC
349
        8.09834494656449805916e+03,  // 0x40BFA2584E6B0563
350
        1.42538291419120476348e+05,  // 0x4101665254D38C3F
351
        8.03309257119514397345e+05,  // 0x412883DA83A52B43
352
        8.40501579819060512818e+05,  // 0x4129A66B28DE0B3D
353
        -3.43899293537866615225e+05, // 0xC114FD6D2C9530C5
354
}
355
 
356
// for x in [8,4.5454]=1/[0.125,0.22001]
357
var q0R5 = [6]float64{
358
        1.84085963594515531381e-11, // 0x3DB43D8F29CC8CD9
359
        7.32421766612684765896e-02, // 0x3FB2BFFFD172B04C
360
        5.83563508962056953777e+00, // 0x401757B0B9953DD3
361
        1.35111577286449829671e+02, // 0x4060E3920A8788E9
362
        1.02724376596164097464e+03, // 0x40900CF99DC8C481
363
        1.98997785864605384631e+03, // 0x409F17E953C6E3A6
364
}
365
var q0S5 = [6]float64{
366
        8.27766102236537761883e+01,  // 0x4054B1B3FB5E1543
367
        2.07781416421392987104e+03,  // 0x40A03BA0DA21C0CE
368
        1.88472887785718085070e+04,  // 0x40D267D27B591E6D
369
        5.67511122894947329769e+04,  // 0x40EBB5E397E02372
370
        3.59767538425114471465e+04,  // 0x40E191181F7A54A0
371
        -5.35434275601944773371e+03, // 0xC0B4EA57BEDBC609
372
}
373
 
374
// for x in [4.547,2.8571]=1/[0.2199,0.35001]
375
var q0R3 = [6]float64{
376
        4.37741014089738620906e-09, // 0x3E32CD036ADECB82
377
        7.32411180042911447163e-02, // 0x3FB2BFEE0E8D0842
378
        3.34423137516170720929e+00, // 0x400AC0FC61149CF5
379
        4.26218440745412650017e+01, // 0x40454F98962DAEDD
380
        1.70808091340565596283e+02, // 0x406559DBE25EFD1F
381
        1.66733948696651168575e+02, // 0x4064D77C81FA21E0
382
}
383
var q0S3 = [6]float64{
384
        4.87588729724587182091e+01,  // 0x40486122BFE343A6
385
        7.09689221056606015736e+02,  // 0x40862D8386544EB3
386
        3.70414822620111362994e+03,  // 0x40ACF04BE44DFC63
387
        6.46042516752568917582e+03,  // 0x40B93C6CD7C76A28
388
        2.51633368920368957333e+03,  // 0x40A3A8AAD94FB1C0
389
        -1.49247451836156386662e+02, // 0xC062A7EB201CF40F
390
}
391
 
392
// for x in [2.8570,2]=1/[0.3499,0.5]
393
var q0R2 = [6]float64{
394
        1.50444444886983272379e-07, // 0x3E84313B54F76BDB
395
        7.32234265963079278272e-02, // 0x3FB2BEC53E883E34
396
        1.99819174093815998816e+00, // 0x3FFFF897E727779C
397
        1.44956029347885735348e+01, // 0x402CFDBFAAF96FE5
398
        3.16662317504781540833e+01, // 0x403FAA8E29FBDC4A
399
        1.62527075710929267416e+01, // 0x403040B171814BB4
400
}
401
var q0S2 = [6]float64{
402
        3.03655848355219184498e+01,  // 0x403E5D96F7C07AED
403
        2.69348118608049844624e+02,  // 0x4070D591E4D14B40
404
        8.44783757595320139444e+02,  // 0x408A664522B3BF22
405
        8.82935845112488550512e+02,  // 0x408B977C9C5CC214
406
        2.12666388511798828631e+02,  // 0x406A95530E001365
407
        -5.31095493882666946917e+00, // 0xC0153E6AF8B32931
408
}
409
 
410
func qzero(x float64) float64 {
411
        var p, q [6]float64
412
        if x >= 8 {
413
                p = q0R8
414
                q = q0S8
415
        } else if x >= 4.5454 {
416
                p = q0R5
417
                q = q0S5
418
        } else if x >= 2.8571 {
419
                p = q0R3
420
                q = q0S3
421
        } else if x >= 2 {
422
                p = q0R2
423
                q = q0S2
424
        }
425
        z := 1 / (x * x)
426
        r := p[0] + z*(p[1]+z*(p[2]+z*(p[3]+z*(p[4]+z*p[5]))))
427
        s := 1 + z*(q[0]+z*(q[1]+z*(q[2]+z*(q[3]+z*(q[4]+z*q[5])))))
428
        return (-0.125 + r/s) / x
429
}

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