Illustration of <a href="https://en.wikipedia.org/wiki/Neighborhood_(mathematics)" class="extiw" title="en:Neighborhood (mathematics)">en:Neighborhood (mathematics)</a>
<a href="//commons.wikimedia.org/wiki/User:Oleg_Alexandrov" title="User:Oleg Alexandrov">Oleg Alexandrov</a>
<span class="int-own-work" lang="en">Own work</span> with <a href="https://en.wikipedia.org/wiki/Inkscape" class="extiw" title="en:Inkscape">en:Inkscape</a>