dbeskn.f90
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real(8) function dbeskn(n,x)
implicit none
! This function compute the rank n Bessel Y function
! using recurrence relation :
! Kn+1(x)=2n/x * Kn(x) + Kn-1(x)
!
integer :: n
real(8) :: x
real(8),external :: dbesk0,dbesk1
real(8) :: two_on_x,bknm1,bkn,bktmp
integer :: i
if (n==0) then
dbeskn=dbesk0(x)
return
end if
if (n==1) then
dbeskn=dbesk1(x)
return
end if
if (n < 0) then
write(*,*) "Error in dbeskn, n must be >= 0"
stop
end if
if (x <= 0.) then
write(*,*) "Error in dbeskn, x must be strictly positive"
end if
two_on_x=2./x
bknm1=dbesk0(x)
bkn=dbesk1(x)
do i=1,n-1
bktmp=two_on_x*bkn*i+bknm1
bknm1=bkn
bkn=bktmp
end do
dbeskn=bktmp
end function