# 12.8: Another Characterization for Planar Graphs

• Eric Lehman, F. Thomson Leighton, & Alberty R. Meyer
• Google and Massachusetts Institute of Technology via MIT OpenCourseWare

We did not pick $$K_5$$ and $$K_{3,3}$$ as examples because of their application to dog houses or quadrapi shaking hands. We really picked them because they provide another, famous, discrete characterizarion of planar graphs:

Theorem $$\PageIndex{1}$$

(Kuratowski). A graph is not planar if and only if it contains $$K_5$$ or $$K_{3,3}$$ as a minor.

Definition $$\PageIndex{2}$$

A minor of a graph $$G$$ is a graph that can be obtained by repeatedly4 deleting vertices, deleting edges, and merging adjacent vertices of $$G$$.

For example, Figure 12.16 illustrates why C3 is a minor of the graph in Figure 12.16(a). In fact C3 is a minor of a connected graph G if and only if G is not a tree. The known proofs of Kuratowski’s Theorem 12.8.1 are a little too long to include in an introductory text, so we won’t give one.

4The three operations can each be performed any number of times in any order.

This page titled 12.8: Another Characterization for Planar Graphs is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Eric Lehman, F. Thomson Leighton, & Alberty R. Meyer (MIT OpenCourseWare) .