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%I #9 May 23 2022 03:55:12
%S 29,43,49,71,87,113,127,129,145,147,197,203,211,213,215,245,261,281,
%T 301,319,337,339,343,355,377,379,381,387,421,435,441,449,473,491,493,
%U 497,539,551,559,565,591,609,631,633,635,637,639,645,673,701,725,731
%N Odd numbers k such that cos(2*Pi/k) is an algebraic number of a 7-smooth degree, but not 5-smooth.
%C A regular polygon of a(n) sides can be constructed if one also has an angle trisector, 5-sector and 7-sector.
%t Do[If[Take[FactorInteger[EulerPhi[2n+1]][[ -1]],1]=={7},Print[2n+1]],{n,1,1000}]
%Y Cf. A125866-A125878.
%K nonn
%O 1,1
%A _Artur Jasinski_, Dec 13 2006
%E Edited by _Don Reble_, Apr 24 2007
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