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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
KEYWORD
nonn,fini,full
AUTHOR
Donald S. McDonald, May 02 2006
STATUS
approved

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Last modified February 26 10:31 EST 2024. Contains 370346 sequences. (Running on oeis4.)