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 A196445 #53 Feb 16 2025 01:00:40
%S A196445 2,6,14,18,22,38,46,54,62,86,94,98,118,134,142,158,162,166,206,214,
%T A196445 242,254,262,278,302,326,334,358,382,398,422,446,454,478,486,502,526,
%U A196445 542,566,614,622,662,686,694,718,722,734,758,766,838,862,878,886,926,934,958,974,982,998
%N A196445 Numbers k >= 2 such that A055035(k) is an odd integer.
%C A196445 All terms are even.
%C A196445 All these cases are so-called reversed cases when degree of minimal polynomial of cos(Pi/n) = 2*degree of minimal polynomial of sin(Pi/n) (in rest of cases is vice versa).
%C A196445 For k = 1, A055035(1) = 1 is also odd. - _Wolfdieter Lang_, Nov 06 2019
%C A196445 The elements of the set {k == 2 (mod 4): k = 2 or phi(k/2)/2 is odd} sorted increasingly, where phi = A000010 (Euler's totient). - _Wolfdieter Lang_, Nov 06 2019
%H A196445 Amiram Eldar, <a href="/A196445/b196445.txt">Table of n, a(n) for n = 1..10000</a>
%F A196445 a(n) = 2*A197504(n).
%t A196445 a[n_] := If[n == 2, 1, EulerPhi[n]/{1, 1, 2, 1}[[Mod[n, 4] + 1]]]; aa = {}; Do[If[OddQ[a[n]], AppendTo[aa, n]], {n, 2, 1000}]; aa
%o A196445 (PARI) isok(k) = ((k%4) == 2) && ((k==2) || (eulerphi(k/2)/2 % 2)==1); \\ after _Wolfdieter Lang_ comment; _Michel Marcus_, Jan 29 2023
%Y A196445 Cf. A000010, A055035, A197504.
%K A196445 nonn,easy
%O A196445 1,1
%A A196445 _Artur Jasinski_, Oct 15 2011
%E A196445 Name made more specific by _Wolfdieter Lang_, Nov 06 2019