Figure 3.

Geometry of the cell ${\mathcal{S}}_0$. Separate expansions are used in the regions ${\mathcal{D}}_1=\{(r,\varphi ):r\in (0,1],\varphi \in [-\pi /N,\pi /N]\}$ and ${\mathcal{D}}_2=\{(r,\varphi ):r\in [1,\mathrm{\infty }),\varphi \in [-\pi /N,\pi /N]\}$.