Because I couldn't count faces and carry out Euler's formula for planar graphs I decided to "find" the $K_{3,3}$ subgraph in this way I'm not sure is how we're supposed to do it.

Given this graph (from here):

enter image description here

I begin by labelling the graph and noting its neighborhoods, I then "merge" three nodes together and get $K_{3,3}$ - is this how its supposed to be done?

I conclude that this graph is not planar.

  • $\begingroup$ Please provide the graph in your post, and tell us which nodes you are identifying with $K_{3,3}$. $\endgroup$
    – Théophile
    May 14, 2015 at 18:52

1 Answer 1


As in the graph you provide, contract $C=\{1,2,3\}$, you will directly get a $K_{3,3}$ with parts $\{C,5,7\}$ and $\{4,6,8\}$. Therefore, it is not planar.


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