code: def _l2deriv(self,l,n):
          
          numer = -3.*nu.sqrt(l) - nu.sqrt(n)
          denom = 4. * l**1.5 * (nu.sqrt(l)+nu.sqrt(n))**3
          return numer / denom
docs: NAME:
              _l2deriv
          PURPOSE:
              evaluate the second derivative w.r.t. lambda for this potential
          INPUT:
              l - prolate spheroidal coordinate lambda
              n - prolate spheroidal coordinate nu
          OUTPUT:
              second derivative w.r.t. lambda
          HISTORY:
              2015-02-15 - Written - Trick (MPIA)