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 A125876 #5 Oct 19 2017 03:15:05
%S A125876 53,79,131,157,313,521,547,677,859,911,937,1093,1171,1249,1301,1873,
%T A125876 1951,2003,2029,2081,2341,2549,2731,2861,3121,3251,3329,3433,3511,
%U A125876 3719,3823,4057,4733,4993,5851,6007,6553,6761,7151,7489,7723,8009
%N A125876 Prime numbers n such that cos(2pi/n) is an algebraic number of a 13-smooth degree, but not 11-smooth.
%t A125876 Do[If[Take[FactorInteger[EulerPhi[2n+1]][[ -1]],1]=={13} && PrimeQ[2n+1],Print[2n+1]],{n,1,10000}] (*Artur Jasinski*)
%Y A125876 Cf. A125866-A125878.
%K A125876 nonn
%O A125876 1,1
%A A125876 _Artur Jasinski_, Dec 13 2006
%E A125876 Edited by _Don Reble_, Apr 24 2007