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%I #9 Jun 01 2019 11:14:33
%S 2,3,5,6,12,20,21,26,33,35,36,42,45,48,50,72,75,77,78,80,90,98,105,
%T 110,120,125,128,132,135,143,147,156,182,192,225,231,252,260,275,288,
%U 297,308,315,330,336,351,363,378,390,392,405,441,450,455,486,500,507,512
%N Numbers n such that p=26n+1 is prime and cos(2pi/p) is an algebraic number of a 13-smooth degree, but not 11-smooth.
%C Numbers n such that p=26n+1 is prime and the greatest prime divisor of p-1 is 13.
%H Harvey P. Dale, <a href="/A125877/b125877.txt">Table of n, a(n) for n = 1..1000</a>
%t Do[If[Take[FactorInteger[EulerPhi[26n+1]][[ -1]],1]=={13} && PrimeQ[26n+1],Print[n]],{n,1,10000}] (*Artur Jasinski*)
%t Select[Range[600],PrimeQ[26#+1]&&FactorInteger[26#][[-1,1]]==13&] (* _Harvey P. Dale_, Jun 01 2019 *)
%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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