which may be drawn from the comparison of cases $N=6$, $12$ and $24$ is the fact, that the energy values of the bottoms and tops of the lower $(-3,0)$ and upper band $(0,3)$ are equal in each case, therefore they are independent of the cluster size. With the growth of the cluster size new energy levels arise, however, previous levels remain the same.

Each energy value in Tab.\[Table1\] is corresponding with some point of the Brillouin zone. More in-detail discussion on that issue is presented in sec.\[sec:Wave functions\]At this point we just briefly discuss the fourfold degeneration of level $\varepsilon=0$ at all $N$.

At first glance it seems not to be in accordance with $\varepsilon$ being 0 at six corners of the Brillouin zone (points $K_1$, $K_2$, …, $K_6$ in Fig.\[Fig3\]).

![Brillouin zone of graphene [@Wallace].[]{data-label="Fig3"}](./Fig3){width="3.5cm"}

However, it must be kept in mind, that only two of these six points, $K_1$ and $K_2$ for example, belong to the first Brillouin zone, others belonging to the next zones. The twofold degeneration at points $K_1$ and $K_2$ is what leads to fourfold degeneration of the level $\varepsilon=0$.

In conclusion of this section it should be noticed, that the rules for diagram construction 