Illustration of <a href="https://en.wikipedia.org/wiki/Neighborhood_(mathematics)" class="extiw" title="en:Neighborhood (mathematics)">Neighborhood</a>
<a href="//commons.wikimedia.org/wiki/User:Oleg_Alexandrov" title="User:Oleg Alexandrov">Oleg Alexandrov</a>
From the source code at <a href="//commons.wikimedia.org/wiki/File:Neighborhood_illust1.png" title="File:Neighborhood illust1.png">File:Neighborhood illust1.png</a>, heavily cleaned-up