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%I #5 Oct 19 2017 03:15:05
%S 2,3,5,8,9,14,15,20,24,27,30,32,35,45,48,50,54,63,72,75,98,105,144,
%T 162,180,189,192,200,224,240,252,300,320,420,450,500,504,525,540,560,
%U 588,630,750,768,864,875,900,960,980,1029,1080,1134,1215,1280,1323
%N Numbers n such that p=14n+1 is prime and cos(2pi/p) is an algebraic number of a 7-smooth degree, but not 5-smooth.
%C Numbers n such that p=14n+1 is prime and the greatest prime divisor of p-1 is 7.
%t Do[If[Take[FactorInteger[EulerPhi[14n+1]][[ -1]],1]=={7} && PrimeQ[14n+1],Print[n]],{n,1,10000}] (*Artur Jasinski*)
%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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