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 A125872 #12 Jun 07 2025 23:56:09
%S A125872 23,67,69,89,115,121,161,199,201,207,253,267,299,331,335,345,353,363,
%T A125872 391,397,437,445,463,469,483,575,597,603,605,617,621,623,661,667,713,
%U A125872 727,737,759,801,805,847,851,871,881,897,943,979,989,991,993,995,1005
%N A125872 Odd numbers k such that cos(2*Pi/k) is an algebraic number of an 11-smooth degree, but not 7-smooth.
%C A125872 A regular polygon of a(n) sides can be constructed if one also has an angle trisector, 5-sector, 7-sector and 11-sector.
%t A125872 Do[If[Take[FactorInteger[EulerPhi[2n+1]][[ -1]],1]=={11},Print[2n+1]],{n,10000}]
%Y A125872 Cf. A125866-A125878.
%K A125872 nonn
%O A125872 1,1
%A A125872 _Artur Jasinski_, Dec 13 2006
%E A125872 Edited by _Don Reble_, Apr 24 2007