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 A125870 #9 May 23 2022 03:55:12
%S A125870 29,43,49,71,87,113,127,129,145,147,197,203,211,213,215,245,261,281,
%T A125870 301,319,337,339,343,355,377,379,381,387,421,435,441,449,473,491,493,
%U A125870 497,539,551,559,565,591,609,631,633,635,637,639,645,673,701,725,731
%N A125870 Odd numbers k such that cos(2*Pi/k) is an algebraic number of a 7-smooth degree, but not 5-smooth.
%C A125870 A regular polygon of a(n) sides can be constructed if one also has an angle trisector, 5-sector and 7-sector.
%t A125870 Do[If[Take[FactorInteger[EulerPhi[2n+1]][[ -1]],1]=={7},Print[2n+1]],{n,1,1000}]
%Y A125870 Cf. A125866-A125878.
%K A125870 nonn
%O A125870 1,1
%A A125870 _Artur Jasinski_, Dec 13 2006
%E A125870 Edited by _Don Reble_, Apr 24 2007