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 A125875 #8 May 23 2022 03:54:47
%S A125875 53,79,131,157,159,169,237,265,313,371,393,395,471,477,507,521,547,
%T A125875 553,583,655,677,689,711,785,795,845,859,869,901,911,917,937,939,1007,
%U A125875 1027,1093,1099,1113,1171,1179,1183,1185,1219,1249,1301,1325,1343
%N A125875 Odd numbers k such that cos(2*Pi/k) is an algebraic number of a 13-smooth degree, but not 11-smooth.
%C A125875 A regular polygon of a(n) sides can be constructed if one also has an angle trisector, 5-, 7-, 11- and 13-sector.
%t A125875 Do[If[Take[FactorInteger[EulerPhi[2n+1]][[ -1]],1]=={13},Print[2n+1]],{n,1,10000}]
%Y A125875 Cf. A125866-A125878.
%K A125875 nonn
%O A125875 1,1
%A A125875 _Artur Jasinski_, Dec 13 2006
%E A125875 Edited by _Don Reble_, Apr 24 2007