+\section{Reciprocal lattice}
+
+Show that the volume $V_{rec}$ of the unit cell of the reciprocal lattice is given by $V_{rec} = (2\pi)^3/V_{real}$, where $V_{real}$ is the volume of the unit cell in real space.
+{\bf Hint:} Use the relation $a \times (b \times c) = b(ac)-c(ab)$ and $a(b \times c)=b(c \times a)$.
+