Bohr-Sommerfield-Wilson quantization condition

According to the Bohr-Sommerfield-Wilson quantization condition, the phase space area enclosed by a periodic path is quantized:

$\frac{1}{2\pi} \oint\! \mathbf{p} \cdot d\mathbf{q} = n \hbar\,$ ,

where $n \in \mathbb{Z}\,$ . Note that the quantity on the left hand side is classically an adiabatic invariant.