International
Tables for Crystallography Volume B Reciprocal space Edited by U. Shmueli © International Union of Crystallography 2006 
International Tables for Crystallography (2006). Vol. B, ch. 3.4, p. 389

If the denominator considered for the limit in the preceding section is linear in H so that only one differentiation is needed to obtain the limit by L'Hospital's method. Since a term of the type is always a factor, the requirement that the unit cell have no dipole moment can be relaxed. For the zerocharge condition is still required: . When the expression becomes determinate and no differentiation is required to obtain a limit. In addition, factoring the sums into sums is not necessary so that the only remaining requirement for this term to be zero is , which is a further relaxation beyond the requirement of cell neutrality.