Isolated Zero
An illustration to support a nice proof by Leo Goldmakher (via Mike Lawler and Dave Radcliffe) of the irrationality of e. It uses the insightful idea that for a rational number q, 0 is isolated in Z+Zq. (Z is the integers.) So if 0 is not isolated in Z+Ze...
Proof at: http://web.williams.edu/Mathematics/lg5/Irrationale.pdf
See also Mike's nice idea for a lesson around this: https://mikesmathpage.wordpress.com/2015/11/30/an-abstract-algebra-question-from-john-golden/