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%I #17 Oct 13 2017 05:11:24
%S 2,3,4,5,16,256,65536
%N Numbers n such that a regular (n^3-n)-gon can be constructed by means of a ruler and compass.
%C Gauss shows a regular n-gon can be constructed (with Euclidean tools) iff n is a product of 2^k*(distinct Fermat primes).
%F n^3 - n = (n-1)*n*(n+1).
%e a(1)=2 because 2^3 - 2 = 6 = 1*2*3, and a regular hexagon can be constructed by ruler and compass.
%o (PARI) for(n=1, 10^4, nn= n^3-n; my(t=eulerphi(nn)); if(t/2^valuation(t, 2)==1, print1(n, ", "))); \\ after PARI in A003401; _Michel Marcus_, Oct 11 2017
%Y Cf. A003401.
%K nonn,fini,full
%O 1,1
%A _Donald S. McDonald_, May 02 2006
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