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 A207333 #10 Apr 09 2021 19:16:51 %S A207333 1,2,3,4,5,6,8,9,10,11,12,14,15,16,18,20,21,22,23,24,26,28,29,30,32, %T A207333 33,35,36,39,27,40,41,42,44,46,48,50,51,52,53,54,56,58,55,60,63,64,65, %U A207333 66,68,69,70,72,74,75,78,81,80,82,83,84,86,88,89,90,92,95 %N A207333 Allowed values of degrees of minimal polynomials of 2*cos(Pi/N). %C A207333 The coefficients of the minimal polynomials C(N,x) of the algebraic number 2*cos(Pi/N) are given in A187360, where N is n. %C A207333 The degree of C(N,x) is delta(N) = 1 if N=1 and delta(N) = phi(2*N)/2 if N > 1, with Euler's totient function phi(n) = A000010(n). %C A207333 The forbidden degree values are shown in the complement (relative to the positive integers) A079695. %C A207333 The array of the values N (the indices) for which the degree delta(N) = a(n), n >= 1, is given in A207334. %F A207333 a(n) gives the allowed degree values, called delta, of the minimal polynomials C ordered increasingly, For C and delta see the comment section. %e A207333 a(8) = 9 because there is at least one polynomial C(N,x) with degree delta(N)=9. In fact the only N values are 19 and 27. %e A207333 7 is no member of this sequence (it belongs to the complement A079695). %Y A207333 Cf. A079695 (complement), A207334 (array of indices of C polynomials with degree a(n)). %K A207333 nonn,easy %O A207333 1,2 %A A207333 _Wolfdieter Lang_, Feb 19 2012