function besjn(n,x,factor,big)

      use fvn_common

      implicit none
      ! This function compute the rank n Bessel J function
      ! using recurrence relation :
      ! Jn+1(x)=2n/x * Jn(x) - Jn-1(x)
      !
      ! Two optional parameters :
      ! factor : an integer that is used in Miller's algorithm to determine the
      ! starting point of iteration. Default value is 40, an increase of this value
      ! will increase accuracy. Starting point ~ nearest even integer of sqrt(factor*n)
      ! big : a real that determine the threshold for taking anti overflow counter measure
      ! default value is 1e10

      !
      real(sp_kind) :: besjn

      integer :: n

      real(sp_kind) :: x

      integer, optional :: factor

      real(sp_kind), optional :: big

  
      integer :: tfactor

      real(sp_kind) :: tbig,tsmall,som
      real(sp_kind),external :: besj0,besj1
      real(sp_kind) :: two_on_x,bjnm1,bjn,bjnp1,absx

      integer :: i,start
      logical :: iseven
  
      ! Initialization of optional parameters
      tfactor=40
      if(present(factor)) tfactor=factor
      tbig=1e10
      if(present(big)) tbig=big
      tsmall=1./tbig
  
      if (n==0) then
          besjn=besj0(x)
          return
      end if
      if (n==1) then
          besjn=besj1(x)
          return
      end if
      if (n < 0) then
          write(*,*) "Error in besjn, n must be >= 0"
          stop
      end if
  
      absx=abs(x)
      if (absx == 0.) then
          besjn=0.

      else if (absx > real(n,sp_kind)) then

      ! For x > n upward reccurence is stable
          two_on_x=2./absx
          bjnm1=besj0(absx)
          bjn=besj1(absx)
          do i=1,n-1
              bjnp1=two_on_x*bjn*i-bjnm1
              bjnm1=bjn
              bjn=bjnp1
          end do
          besjn=bjnp1
      else
          ! For x <= n we use Miller's Algorithm
          ! as upward reccurence is unstable.
          ! This is adapted from Numerical Recipes
          ! Principle : use of downward recurrence from an arbitrary
          ! higher than n value with an arbitrary seed,
          ! and then use the normalization formula :
          ! 1=J0+2J2+2J4+2J6+....
          two_on_x=2./absx

          start=2*((n+int(sqrt(real(n*tfactor,sp_kind))))/2) ! even start

          som=0.
          iseven=.false.
          bjnp1=0.
          bjn=1.
          do i=start,1,-1
              ! begin downward rec
              bjnm1=two_on_x*bjn*i-bjnp1
              bjnp1=bjn
              bjn=bjnm1
              ! Action to prevent overflow
              if (abs(bjn) > tbig) then
                  bjn=bjn*tsmall
                  bjnp1=bjnp1*tsmall
                  besjn=besjn*tsmall
                  som=som*tsmall
              end if
              if (iseven) then
                  som=som+bjn
              end if
              iseven= .not. iseven
              if (i==n) besjn=bjnp1
          end do
          som=2.*som-bjn
          besjn=besjn/som
      end if
      ! if n is odd and x <0
      if ((x<0.) .and. (mod(n,2)==1)) besjn=-besjn
  
  end function