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 A216647 #16 Mar 22 2020 04:01:28
%S A216647 2,3,2,4,3,3,4,5,4,4,6,4,7,5,5,6,5,5,10,5,7,7,12,5,11,8,10,6,15,6,6,7,
%T A216647 6,6,13,6,19,11,13,6,11,8,8,8,13,13,24,6,22,12,9,9,27,11,21,7,10,16,
%U A216647 30,7,31,7,7,8,7,7,34,7,23,14,36,7
%N A216647 a(n) := card{cos((2^(k-1))*Pi/n): k=1,2,...}.
%C A216647 The sequence a(n) is an "even" supplement of the sequence A216066.
%C A216647 Does there exist an infinite set of solutions (in indices n in N) to each of the following three relations: a(n) + a(n+2) > a(n+1), a(n) + a(n+2) = a(n+1), and a(n) + a(n+2) < a(n+1)?
%H A216647 R. Witula and D. Slota, <a href="https://doi.org/10.1142/S021812740902461X">Fixed and periodic points of polynomials generated by minimal polynomials of 2cos(2Pi/n)</a>, International J. Bifurcation and Chaos, 19 (9) (2009), 3005.
%Y A216647 Cf. A216066.
%K A216647 nonn
%O A216647 1,1
%A A216647 _Roman Witula_, Sep 12 2012