-
git-svn-id: https://lxsd.femto-st.fr/svn/fvn@59 b657c933-2333-4658-acf2-d3c7c2708721
beskn.f90
878 Bytes
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
function beskn(n,x)
use fvn_common
implicit none
! This function compute the rank n Bessel Y function
! using recurrence relation :
! Kn+1(x)=2n/x * Kn(x) + Kn-1(x)
!
real(sp_kind) :: beskn
integer :: n
real(sp_kind) :: x
real(sp_kind),external :: besk0,besk1
real(sp_kind) :: two_on_x,bknm1,bkn,bktmp
integer :: i
if (n==0) then
beskn=besk0(x)
return
end if
if (n==1) then
beskn=besk1(x)
return
end if
if (n < 0) then
write(*,*) "Error in beskn, n must be >= 0"
stop
end if
if (x <= 0.) then
write(*,*) "Error in beskn, x must be strictly positive"
end if
two_on_x=2./x
bknm1=besk0(x)
bkn=besk1(x)
do i=1,n-1
bktmp=two_on_x*bkn*i+bknm1
bknm1=bkn
bkn=bktmp
end do
beskn=bktmp
end function