code: def _lnderiv(self,l,n):
          
          return -0.5/(nu.sqrt(l) * nu.sqrt(n) * (nu.sqrt(l)+nu.sqrt(n))**3)
docs: NAME:
              _lnderiv
          PURPOSE:
              evaluate the mixed derivative w.r.t. lambda and nu for this potential
          INPUT:
              l - prolate spheroidal coordinate lambda
              n - prolate spheroidal coordinate nu
          OUTPUT:
              d2phi/dl/dn
          HISTORY:
              2015-02-13 - Written - Trick (MPIA)