# How to prove that a simple graph having 11 or more vertices or its complement is not planar?

It is an exercise on a book again.If a simple graph G has 11 or more vertices,then either G or is complement $\bar { G }$ is not planar. How to begin with this?Induction? Thanks for your help!

• Maybe using Kuratowki's theorem. – Josué Tonelli-Cueto Apr 6 '12 at 10:54
• Or maybe Euler's formula. What is the maximum number of edges that a simple planar graph with 11 vertices can have? – Peter Shor Apr 6 '12 at 10:55

## 1 Answer

It follows from the Euler's formula that a simple planar graph $G$ with $m$ edges and $n\geq 3$ vertices must satisfy (see here) $$\tag{1}m\leq 3n-6.$$ For a graph $G$ with $m$ edges and $n$ vertices, its complement $\overline{G}$ has $\displaystyle\frac{n(n-1)}{2}-m$ edges. Therefore, if $\overline{G}$ is also planar, by $(1)$ we have $$\tag{2}\frac{n(n-1)}{2}-m\leq 3n-6.$$ Adding $(1)$ and $(2)$, we obtain $$\frac{n(n-1)}{2}\leq 6n-12,$$ which implies that $n\leq 10$.

• Oh,the book I am reading now doesn't have this formula m≤3n−6 ... Now it seems not to be a good book. – tamlok Apr 7 '12 at 1:49
• I've had this doubt, could it be that both $G \text{ and } \overline G$ are not planar? – YoTengoUnLCD Sep 13 '15 at 20:13
• @YoTengoUnLCD Of course. What this proved was that at least one of $G$ and $\overline{G}$ must be nonplanar. If you want specific constructions of graphs who are nonplanar whose complements are also nonplanar, see for example this question. – JMoravitz May 25 '16 at 17:46