A 3-regular <a href="https://en.wikipedia.org/wiki/Planar_graph" class="extiw" title="en:Planar graph">planar graph</a> that requires four colors in any <a href="https://en.wikipedia.org/wiki/Edge_coloring" class="extiw" title="en:Edge coloring">edge coloring</a>
<a href="//commons.wikimedia.org/wiki/User:David_Eppstein" class="mw-redirect" title="User:David Eppstein">David Eppstein</a>
<span class="int-own-work" lang="en">Own work</span>