The OEIS is supported by the many generous donors to the OEIS Foundation.
%I #9 May 23 2022 03:55:22
%S 23,67,69,89,115,121,161,199,201,207,253,267,299,331,335,345,353,363,
%T 391,397,437,445,463,469,483,575,597,603,605,617,621,623,661,667,713,
%U 727,737,759,801,805,847,851,871,881,897,943,979,989,991,993,995,1005
%N Odd numbers k such that cos(2*Pi/k) is an algebraic number of an 11-smooth degree, but not 7-smooth.
%C 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 Do[If[Take[FactorInteger[EulerPhi[2n+1]][[ -1]],1]=={11},Print[2n+1]],{n,1,10000}]]
%Y Cf. A125866-A125878.
%K nonn
%O 1,1
%A _Artur Jasinski_, Dec 13 2006
%E Edited by _Don Reble_, Apr 24 2007