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dbskes.f
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subroutine dbskes (xnu, x, nin, bke)
c july 1980 edition. w. fullerton, c3, los alamos scientific lab.
double precision xnu, x, bke(*), bknu1, v, vincr, vend, alnbig,
1 d1mach, dlog
external d1mach
data alnbig / 0.d0 /
c
if (alnbig.eq.0.d0) alnbig = dlog (d1mach(2))
c
v = dabs(xnu)
n = iabs(nin)
c
if (v.ge.1.d0) call seteru (29hdbskes abs(xnu) must be lt 1, 29,
1 1, 2)
if (x.le.0.d0) call seteru (17hdbskes x is le 0, 17, 2, 2)
if (n.eq.0) call seteru (
1 41hdbskes n the number in the sequence is 0, 41, 3, 2)
c
call d9knus (v, x, bke(1), bknu1, iswtch)
if (n.eq.1) return
c
vincr = sign (1.0, float(nin))
direct = vincr
if (xnu.ne.0.d0) direct = vincr*dsign(1.d0, xnu)
if (iswtch.eq.1 .and. direct.gt.0.) call seteru (
1 47hdbskes x so small bessel k-sub-xnu+1 overflows, 47, 4, 2)
bke(2) = bknu1
c
if (direct.lt.0.) call d9knus (dabs(xnu+vincr), x, bke(2), bknu1,
1 iswtch)
if (n.eq.2) return
c
vend = dabs (xnu+dble(float(nin))) - 1.0d0
if (x + (vend-.5d0)*dlog(vend)+0.27d0-vend*(dlog(x)+.306d0)
1 .gt.alnbig) call seteru (
2 67hdbskes x so small or abs(nu) so big that bessel
3 k-sub-nu overflows, 67, 5, 2)
c
v = xnu
do 10 i=3,n
v = v + vincr
bke(i) = 2.0d0*v*bke(i-1)/x + bke(i-2)
10 continue
c
return
end