The butterfly and diamond graphs, forbidden <a href="https://en.wikipedia.org/wiki/Graph_minor" class="extiw" title="en:Graph minor">minors</a> for <a href="https://en.wikipedia.org/wiki/Pseudoforest" class="extiw" title="en:Pseudoforest">pseudoforests</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>