Reciprocal-lattice peaks of a finite crystal (source code)

= Reciprocal-lattice peaks of a finite crystal

For $S=\{\sum_il_ia_i:-L_i/2\leq l_i\leq L_i/2\}$ and $\alpha_i=Q\cdot a_i$,
$$
\Delta(Q)=\prod_{i=1}^3
\frac{\sin((L_i+1)\alpha_i/2)}{\sin(\alpha_i/2)}.
$$
For large $L_i$, each factor is sharply peaked when $\alpha_i\in2\pi\mathbb Z$, exactly the condition $Q\in\Lambda^*$.