This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A125877 #16 Aug 02 2025 13:17:01
%S A125877 2,3,5,6,12,20,21,26,33,35,36,42,45,48,50,72,75,77,78,80,90,98,105,
%T A125877 110,120,125,128,132,135,143,147,156,182,192,225,231,252,260,275,288,
%U A125877 297,308,315,330,336,351,363,378,390,392,405,441,450,455,486,500,507,512
%N A125877 Numbers k such that p=26*k+1 is prime and cos(2*Pi/p) is an algebraic number of a 13-smooth degree, but not 11-smooth.
%C A125877 Numbers k such that p=26*k+1 is prime and the greatest prime divisor of p-1 is 13.
%H A125877 Harvey P. Dale, <a href="/A125877/b125877.txt">Table of n, a(n) for n = 1..1000</a>
%t A125877 Do[If[Take[FactorInteger[EulerPhi[26n+1]][[ -1]],1]=={13} && PrimeQ[26n+1],Print[n]],{n,1,10000}]
%t A125877 (* or *)
%t A125877 Select[Range[600],PrimeQ[26#+1]&&FactorInteger[26#][[-1,1]]==13&] (* _Harvey P. Dale_, Jun 01 2019 *)
%Y A125877 Cf. A125866-A125878.
%K A125877 nonn
%O A125877 1,1
%A A125877 _Artur Jasinski_, Dec 13 2006
%E A125877 Edited by _Don Reble_, Apr 24 2007