(1) The most usual setting is in reciprocal space (see Sayre, 1952c). Only a finite number of diffraction intensities can be recorded and phased, and for physical reasons the cutoff criterion is the resolution . Electrondensity maps are thus calculated as partial sums (Section 1.3.4.2.1.3), which may be written in Cartesian coordinates as is bandlimited, the support of its spectrum being contained in the solid sphere defined by . Let be the indicator function of . The transform of the normalized version of is (see below, Section 1.3.4.4.3.5) By Shannon's theorem, it suffices to calculate on an integral subdivision Γ of the period lattice Λ such that the sampling criterion is satisfied (i.e. that the translates of by vectors of do not overlap). Values of may then be calculated at an arbitrary point X by the interpolation formula:
(2) The reverse situation occurs whenever the support of the motif does not fill the whole unit cell, i.e. whenever there exists a region M (the `molecular envelope'), strictly smaller than the unit cell, such that the translates of M by vectors of r do not overlap and that It then follows that Defining the `interference function' G as the normalized indicator function of M according to we may invoke Shannon's theorem to calculate the value at an arbitrary point ξ of reciprocal space from its sample values at points of the reciprocal lattice as This aspect of Shannon's theorem constitutes the mathematical basis of phasing methods based on geometric redundancies created by solvent regions and/or noncrystallographic symmetries (Bricogne, 1974). The connection between Shannon's theorem and the phase problem was first noticed by Sayre (1952b). He pointed out that the Patterson function of , written as , may be viewed as consisting of a motif (containing all the internal interatomic vectors) which is periodized by convolution with r. As the translates of by vectors of do overlap, the sample values of the intensities at nodes of the reciprocal lattice do not provide enough data to interpolate intensities at arbitrary points of reciprocal space. Thus the loss of phase is intimately related to the impossibility of intensity interpolation, implying in return that any indication of intensity values attached to nonintegral points of the reciprocal lattice is a potential source of phase information.
