subroutine POISUP (n, beta, nu_up, ierr)

c  Author:  Glen Cowan
c  Date:    24-Jan-1998

c  Computes the classical upper limit at a confidence level cl=1-beta 
c  for the Poisson parameter nu given the observed number n.
c  Must be linked with the routine CHISIN (quantile of chi^2 distribution)
c  from the CERN Program Library.

      implicit        NONE

c  arguments

      integer         ierr      ! 0 = ok
      integer         n

      real            beta
      real            nu_up

c  local variables

      integer         n_dof

      real            CHISIN
      real            cl

c  begin

      cl = 1.-beta
      ierr = 0
      if ( n .lt. 0 ) then
        ierr = 1
      elseif ( cl .lt. 0. ) then
        ierr = 2
      elseif ( cl .gt. 1. ) then
        ierr = 3
      endif
      if ( ierr .ne. 0 ) then
        nu_up = -1.
        return   ! ---------------------> exit
      endif

      n_dof = 2*(n+1)
      nu_up = 0.5 * CHISIN (cl, n_dof)

      return
      END