besyn.f90
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real(4) function besyn(n,x)
implicit none
! This function compute the rank n Bessel Y function
! using recurrence relation :
! Yn+1(x)=2n/x * Yn(x) - Yn-1(x)
!
integer :: n
real(4) :: x
real(4),external :: besy0,besy1
real(4) :: two_on_x,bynm1,byn,bytmp
integer :: i
if (n==0) then
besyn=besy0(x)
return
end if
if (n==1) then
besyn=besy1(x)
return
end if
if (n < 0) then
write(*,*) "Error in besyn, n must be >= 0"
stop
end if
if (x <= 0.) then
write(*,*) "Error in besyn, x must be strictly positive"
end if
two_on_x=2./x
bynm1=besy0(x)
byn=besy1(x)
do i=1,n-1
bytmp=two_on_x*byn*i-bynm1
bynm1=byn
byn=bytmp
end do
besyn=bytmp
end function