real(4) function beskn(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(4) :: x
  
      real(4),external :: besk0,besk1
      real(4) :: 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