The Calkin–Wilf sequence, a sequence that includes each positive rational number exactly once and is formed by breadth first traversal of the <a href="https://en.wikipedia.org/wiki/Calkin%E2%80%93Wilf_tree" class="extiw" title="en:Calkin–Wilf tree">Calkin–Wilf tree</a>. The sequence is depicted as the red spiral tracing through the tree.
<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>