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 A117885 Numbers n such that a regular (n^3-n)-gon can be constructed by means of a ruler and compass. 0
 2, 3, 4, 5, 16, 256, 65536 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Gauss shows a regular n-gon can be constructed (with Euclidean tools) iff n is a product of 2^k*(distinct Fermat primes). LINKS FORMULA n^3 - n = (n-1)*n*(n+1). EXAMPLE a(1)=2 because 2^3 - 2 = 6 = 1*2*3, and a regular hexagon can be constructed by ruler and compass. PROG (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 CROSSREFS Cf. A003401. Sequence in context: A293824 A095181 A240906 * A030574 A325652 A283653 Adjacent sequences:  A117882 A117883 A117884 * A117886 A117887 A117888 KEYWORD nonn,fini,full AUTHOR Donald S. McDonald, May 02 2006 STATUS approved

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Last modified December 5 04:10 EST 2021. Contains 349530 sequences. (Running on oeis4.)