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%I #77 Jan 28 2020 17:04:39
%S -6,-4,-3,-2,-1,1,2,3,4,6
%N Integers n such that 2*cos(2*Pi/n) is an integer.
%C Terms are the allowable n-fold rotational symmetries of a crystal (rotation by 360 degrees/n leaves the object unchanged).
%C 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 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 Wikipedia, <a href="http://en.wikipedia.org/wiki/Crystallographic_restriction_theorem">Crystallographic Restriction Theorem</a>
%e 2*cos(2Pi/1) = 2
%e 2*cos(2Pi/2) = -2
%e 2*cos(2Pi/3) = -1
%e 2*cos(2Pi/4) = 0
%e 2*cos(2Pi/6) = 1
%e 2*cos(2Pi/10) = 1.6180339887... and so 10, for instance, is not in this sequence.
%K sign,fini,full
%O 0,1
%A _Raphie Frank_, Sep 30 2012
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