The extension concept and its role in the probabilistic theory of the structure seminvariants

By embedding the structure seminvariant T and symmetry-related variants of T in suitable structure invariants Q the values of which, because of the space-group-dependent relations among the phases, are related to T, one reduces the probabilistic theory of the structure seminvariants to that of the structure invariants, which is well developed. The structure invariants Q are said to be extensions of the structure seminvariant T.