For visualization purposes, we might want to get an RDF on a grid in r-space with steps below what is allowed by the Nyquist-Shannon-Kotelnikov theorem (see Sec. II B). In Ref. , such limit was erroneously 56,57 denoted as the "diffraction limit". In this case, the existence of the matrix Σ 0 = (S T W S ) -1 is not guaranteed, or large correlations (Eq. 17) between values can arise. In this case, we can apply the L2-regularization (or ridge regression) to obtain a stable solution similar to Equation .