Set $\mathbf k=\mathbf h$ in the <direct-methods triplet relationship>. It becomes $S(\mathbf h)^2S(2\mathbf h)\simeq+1$, and since the square of a nonzero sign is one, $S(2\mathbf h)\simeq+1$. This recovers the <direct-methods pair relationship>, with the same strong-amplitude qualifications.

Solution

Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 4 / 58 / b / v / Solution

 Solution (source code)