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double precision function dbesi0 (x)
c july 1977 edition. w. fullerton, c3, los alamos scientific lab.
double precision x, bi0cs(18), xmax, xsml, y, d1mach,
1 dcsevl, dbsi0e, dexp, dlog, dsqrt
external d1mach, dbsi0e, dcsevl, initds
c
c series for bi0 on the interval 0. to 9.00000e+00
c with weighted error 9.51e-34
c log weighted error 33.02
c significant figures required 33.31
c decimal places required 33.65
c
data bi0 cs( 1) / -.7660547252 8391449510 8189497624 3285 d-1 /
data bi0 cs( 2) / +.1927337953 9938082699 5240875088 1196 d+1 /
data bi0 cs( 3) / +.2282644586 9203013389 3702929233 0415 d+0 /
data bi0 cs( 4) / +.1304891466 7072904280 7933421069 1888 d-1 /
data bi0 cs( 5) / +.4344270900 8164874513 7868268102 6107 d-3 /
data bi0 cs( 6) / +.9422657686 0019346639 2317174411 8766 d-5 /
data bi0 cs( 7) / +.1434006289 5106910799 6209187817 9957 d-6 /
data bi0 cs( 8) / +.1613849069 6617490699 1541971999 4611 d-8 /
data bi0 cs( 9) / +.1396650044 5356696994 9509270814 2522 d-10 /
data bi0 cs( 10) / +.9579451725 5054453446 2752317189 3333 d-13 /
data bi0 cs( 11) / +.5333981859 8625021310 1510774400 0000 d-15 /
data bi0 cs( 12) / +.2458716088 4374707746 9678591999 9999 d-17 /
data bi0 cs( 13) / +.9535680890 2487700269 4434133333 3333 d-20 /
data bi0 cs( 14) / +.3154382039 7214273367 8933333333 3333 d-22 /
data bi0 cs( 15) / +.9004564101 0946374314 6666666666 6666 d-25 /
data bi0 cs( 16) / +.2240647369 1236700160 0000000000 0000 d-27 /
data bi0 cs( 17) / +.4903034603 2428373333 3333333333 3333 d-30 /
data bi0 cs( 18) / +.9508172606 1226666666 6666666666 6666 d-33 /
c
data nti0, xsml, xmax / 0, 2*0.d0 /
c
if (nti0.ne.0) go to 10
nti0 = initds (bi0cs, 18, 0.1*sngl(d1mach(3)))
xsml = dsqrt (8.0d0*d1mach(3))
xmax = dlog (d1mach(2))
c
10 y = dabs(x)
if (y.gt.3.0d0) go to 20
c
dbesi0 = 1.0d0
if (y.gt.xsml) dbesi0 = 2.75d0 + dcsevl (y*y/4.5d0-1.d0, bi0cs,
1 nti0)
return
c
20 if (y.gt.xmax) call seteru (
1 35hdbesi0 dabs(x) so big i0 overflows, 35, 2, 2)
c
dbesi0 = dexp(y) * dbsi0e(x)
c
return
end
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