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double precision function dbesi1 (x)
c oct 1983 edition. w. fullerton, c3, los alamos scientific lab.
double precision x, bi1cs(17), xmax, xmin, xsml, y, d1mach,
1 dcsevl, dbsi1e, dexp, dlog, dsqrt
external d1mach, dbsi1e, dcsevl, initds
c
c series for bi1 on the interval 0. to 9.00000e+00
c with weighted error 1.44e-32
c log weighted error 31.84
c significant figures required 31.45
c decimal places required 32.46
c
data bi1 cs( 1) / -.1971713261 0998597316 1385032181 49 d-2 /
data bi1 cs( 2) / +.4073488766 7546480608 1553936520 14 d+0 /
data bi1 cs( 3) / +.3483899429 9959455866 2450377837 87 d-1 /
data bi1 cs( 4) / +.1545394556 3001236038 5984010584 89 d-2 /
data bi1 cs( 5) / +.4188852109 8377784129 4588320041 20 d-4 /
data bi1 cs( 6) / +.7649026764 8362114741 9597039660 69 d-6 /
data bi1 cs( 7) / +.1004249392 4741178689 1798080372 38 d-7 /
data bi1 cs( 8) / +.9932207791 9238106481 3712980548 63 d-10 /
data bi1 cs( 9) / +.7663801791 8447637275 2001716813 49 d-12 /
data bi1 cs( 10) / +.4741418923 8167394980 3880919481 60 d-14 /
data bi1 cs( 11) / +.2404114404 0745181799 8631720320 00 d-16 /
data bi1 cs( 12) / +.1017150500 7093713649 1211007999 99 d-18 /
data bi1 cs( 13) / +.3645093565 7866949458 4917333333 33 d-21 /
data bi1 cs( 14) / +.1120574950 2562039344 8106666666 66 d-23 /
data bi1 cs( 15) / +.2987544193 4468088832 0000000000 00 d-26 /
data bi1 cs( 16) / +.6973231093 9194709333 3333333333 33 d-29 /
data bi1 cs( 17) / +.1436794822 0620800000 0000000000 00 d-31 /
c
data nti1, xmin, xsml, xmax / 0, 3*0.d0 /
c
if (nti1.ne.0) go to 10
nti1 = initds (bi1cs, 17, 0.1*sngl(d1mach(3)))
xmin = 2.0d0*d1mach(1)
xsml = dsqrt (8.0d0*d1mach(3))
xmax = dlog (d1mach(2))
c
10 y = dabs(x)
if (y.gt.3.0d0) go to 20
c
dbesi1 = 0.0d0
if (y.eq.0.0d0) return
c
if (y.le.xmin) call seteru (
1 38hdbesi1 dabs(x) so small i1 underflows, 38, 1, 0)
if (y.gt.xmin) dbesi1 = 0.5d0*x
if (y.gt.xsml) dbesi1 = x*(0.875d0 + dcsevl (y*y/4.5d0-1.d0,
1 bi1cs, nti1))
return
c
20 if (y.gt.xmax) call seteru (
1 35hdbesi1 dabs(x) so big i1 overflows, 35, 2, 2)
c
dbesi1 = dexp(y) * dbsi1e(x)
c
return
end
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