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 A210845 #7 Apr 11 2012 13:09:43 %S A210845 1,2,3,4,5,6,7,9,11,13,14,18,21,22,23,25,29,31,33,43,46,47,49,53,59, %T A210845 61,62,67,69,71,77,79,83,86,93,94,98,99,103,107,118,121,129,131,134, %U A210845 139,141,142,147,149,157,158,161,166,167,169,173,177,179,191 %N A210845 Values n for which A055034(n) is squarefree. %C A210845 A055034(n) is the degree delta(n) of the minimal polynomial of the algebraic number rho(n):=2*cos(pi/n), n>=1, whose coefficients are shown in A187360. It is also the order of multiplicative abelian group Modd n (for multiplication Modd n see a comment on A203571). This is the Galois group Gal(Q(rho(n))/Q). If the number of abelian groups of order delta(n) is 1 then this group is necessarily cyclic. %C A210845 Because A000688 is 1 exactly for the squarefree numbers A005117, the set of a(n) values of the present sequence is a (proper) subset of A206551. Hence it is immediately clear that the multiplicative group Modd a(n) is cyclic, but there are other cyclic Modd n groups, e.g., for n = 8, 10, 15, 16, 17, 19, 26, 27, 32, 34, 35, 37, 38, 39, 41,... %F A210845 A055034(a(n)) is squarefree, i.e. from A005117. %e A210845 a(3)=3 because delta(3)=A055034(3)= 1, and 1 is a member of the squarefree numbers A005117. %e A210845 a(8)=9 because A055034(9)= 3 = A005117(3). %e A210845 a(10)=13 because A055034(13)= 6 = A005117(5). %Y A210845 Cf. A206551. %K A210845 nonn,easy %O A210845 1,2 %A A210845 _Wolfdieter Lang_, Apr 11 2012