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. 1.3, p. 69

A formula for the Fourier synthesis of electrondensity maps from symmetryunique structure factors is readily obtained by orbit decomposition: where L is a subset of such that contains exactly one point of each orbit for the action of G on . The physical electron density per cubic ångström is then with V in Å^{3}.
In the absence of anomalous scatterers in the crystal and of a centre of inversion −I in Γ, the spectrum has an extra symmetry, namely the Hermitian symmetry expressing Friedel's law (Section 1.3.4.2.1.4). The action of a centre of inversion may be added to that of Γ to obtain further simplification in the above formula: under this extra action, an orbit with is either mapped into itself or into the disjoint orbit ; the terms corresponding to and may then be grouped within the common orbit in the first case, and between the two orbits in the second case.