International
Tables for Crystallography Volume B Reciprocal space Edited by U. Shmueli © International Union of Crystallography 2010 
International Tables for Crystallography (2010). Vol. B, ch. 1.3, pp. 7879

Suppose that the spacegroup action is reducible, i.e. that for each by Schur's lemma, the decimation matrix must then be of the formif it is to commute with all the .
Puttingwe may defineand write (direct sum) as a shorthand for
We may also define the representation operators and acting on functions of and , respectively (as in Section 1.3.4.2.2.4), and the operators and acting on functions of and , respectively (as in Section 1.3.4.2.2.5). Then we may writeandin the sense that g acts on byand on by
Thus equipped we may now derive concisely a general identity describing the symmetry properties of intermediate quantities of the formwhich arise through partial transformation of F on or of on . The action of on these quantities will be
and hence the symmetry properties of T are expressed by the identityApplying this relation not to T but to givesi.e.
If the unique were initially indexed by(see Section 1.3.4.2.2.2), this formula allows the reindexing of the intermediate results from the initial formto the final formon which the second transform (on ) may now be performed, giving the final results indexed bywhich is an asymmetric unit. An analogous interpretation holds if one is going from to F.
The above formula solves the general problem of transposing from one invariant subspace to another, and is the main device for decomposing the CDFT. Particular instances of this formula were derived and used by Ten Eyck (1973); it is useful for orthorhombic groups, and for dihedral groups containing screw axes with g.c.d. . For comparison with later uses of orbit exchange, it should be noted that the type of intermediate results just dealt with is obtained after transforming on all factors in one summand.
A central piece of information for driving such a decomposition is the definition of the full asymmetric unit in terms of the asymmetric units in the invariant subspaces. As indicated at the end of Section 1.3.4.2.2.2, this is straightforward when G acts without fixed points, but becomes more involved if fixed points do exist. To this day, no systematic `calculus of asymmetric units' exists which can automatically generate a complete description of the asymmetric unit of an arbitrary space group in a form suitable for directing the orbit exchange process, although Shenefelt (1988) has outlined a procedure for dealing with space group P622 and its subgroups. The asymmetric unit definitions given in Volume A of International Tables are incomplete in this respect, in that they do not specify the possible residual symmetries which may exist on the boundaries of the domains.
References
Shenefelt, M. (1988). Group Invariant Finite Fourier Transforms. PhD thesis, Graduate Centre of the City University of New York.Ten Eyck, L. F. (1973). Crystallographic fast Fourier transforms. Acta Cryst. A29, 183–191.