# Is my understanding of when a graph is planar (or not) correct?

Is the following correct?

A graph is planar when the same graph can be drawn without any edge crossings.

The only way to show that a graph is planar is to draw a planar representation of it.

A graph is not planar when:

1. $e \le 3v - 6$ is false ($v \ge 3$)
2. $e \le 2v - 4$ is false (The graph has no cycle of length $3$ and $v \ge 3$)
3. The graph is nonplanar iff it contains a subgraph homeomorphic to $K_{3,3}$ or $K_{5}$ (Kuratowski's Theorem)

However, if (1) and (2) are true then we cannot say that the graph is planar. For the third condition, we have to first find a subgraph then proceed to show through elementary subdivisions that the subgraph is a homeomorphism to $K_{3,3}$ or $K_{5}$

Is this understanding correct?

• By Euler's formula ($v-e+f=2$), couldn't we say that (2) supersedes (1) making (1) unnecessary? – Eli Sadoff Dec 7 '16 at 17:57
• @EliSadoff Yes, I believe that (2) is found in a similar way to (1). However, there is an extra condition i.e. "no cycle of length 3" which helps show that $K_{3,3}$ is non-planar which (1) cannot do. – Jeel Shah Dec 7 '16 at 17:58
Yeah, everything you've written seems correct. I don't like your line, "The only way to show that a graph is planar is to draw a planar representation of it." Certainly that is the easiest way, but not the only way. You could show that a graph does not have a subgraph homeomorphic to $K_5$ or $K_{3,3}$ to show it is planar.