This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A217290 #77 Jan 28 2020 17:04:39
%S A217290 -6,-4,-3,-2,-1,1,2,3,4,6
%N A217290 Integers n such that 2*cos(2*Pi/n) is an integer.
%C A217290 Terms are the allowable n-fold rotational symmetries of a crystal (rotation by 360 degrees/n leaves the object unchanged).
%C A217290 The positive values of this sequence {1, 2, 3, 4, 6} are the proper divisors of 12, all having a totient of 1 or 2 (see A000010).
%H A217290 W. Scherrer, <a href="https://gdz.sub.uni-goettingen.de/id/PPN378850199_0001?tify={%22pages%22:[101]}">Die Einlagerung eines regulären Vielecks in ein Gitter</a>, Elemente der Mathematik, 1946, 1(6), p.97-98.
%H A217290 Wikipedia, <a href="http://en.wikipedia.org/wiki/Crystallographic_restriction_theorem">Crystallographic Restriction Theorem</a>
%e A217290 2*cos(2Pi/1) = 2
%e A217290 2*cos(2Pi/2) = -2
%e A217290 2*cos(2Pi/3) = -1
%e A217290 2*cos(2Pi/4) = 0
%e A217290 2*cos(2Pi/6) = 1
%e A217290 2*cos(2Pi/10) = 1.6180339887... and so 10, for instance, is not in this sequence.
%K A217290 sign,fini,full
%O A217290 0,1
%A A217290 _Raphie Frank_, Sep 30 2012