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 A215046 #10 Jan 15 2018 15:33:12 %S A215046 4,5,6,7,9,11,23,47,59,83,107,167,179,227,263,347,359,383,467,479,503, %T A215046 563,587,719,839,863,887,983,1019,1187,1283,1307,1319,1367,1439,1487, %U A215046 1523,1619,1823,1907,2027,2039,2063,2099,2207,2447,2459 %N A215046 Increasingly ordered list of those values m for which the degree of the minimal polynomial of 2*cos(Pi/m) (see A187360) is prime. %C A215046 The degree delta(m) of the minimal polynomial of rho(m) := 2*cos(Pi/m), called C(m,x) with coefficient array A187360, is given by A055034(m). %C A215046 If delta(m) = phi(2*m)/2, m>=2, delta(1) = 1, with phi = A000010, is prime then the (Abelian) Galois group G(Q(rho(m))/Q) is cyclic. Because this Galois group of C(m,x) has order delta(m) this follows from a corollary to Lagrange's theorem, or also from Cauchy's theorem on groups. %C A215046 Because the mentioned Galois group is isomorphic to the multiplicative group Modd m of order delta(m) (see a comment on A203571) all m = a(n) values appear in A206551. %C A215046 This sequence is also a subsequence of A210845 because p is squarefree (see A005117). %F A215046 phi(2*m)/2 is prime iff m=a(n), n>=1, with phi = A000010 (Euler's totient). %e A215046 a(4) = 7, because 7 satisfies phi(14)/2 = phi(2*7)/2 = 1*6/2 = 3, which is prime; and 7 is the fourth smallest number m satisfying: phi(2*m)/2 is prime. %Y A215046 Cf. A055034. %K A215046 nonn %O A215046 1,1 %A A215046 _Wolfdieter Lang_, Sep 03 2012