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 A115123 #6 Aug 28 2019 08:15:47
%S A115123 0,1,1,2,1,5,1,9,4,22,1,63,1,156,24,469,1,1532,1,4889,158,16546,1,
%T A115123 57844,21,202061,1480,720533,1,2598406,1,9408469,16548,34350508,175,
%U A115123 126167112,1,465200334,202063,1723346534,1,6409076632,1,23911272097,2594970,89495909410,1
%N A115123 Number of imprimitive (periodic) bracelets (or necklaces) with n red and blue beads such that the beads switch colors when bracelet is turned over.
%C A115123 a(p)=1 for prime p.
%F A115123 a(n)=A006080(n+1) - A066315(n).
%t A115123 A006080[n_] := Sum[EulerPhi[(n - 1)/k] (Binomial[2k, k]/(2(n - 1))), {k, Divisors[n - 1]}]/2 + 2^(n - 3); A006080[1] = 1;
%t A115123 A066315[n_] := Sum[MoebiusMu[n/d] A006080[d + 1], {d, Divisors[n]}];
%t A115123 a[n_] := A006080[n + 1] - A066315[n];
%t A115123 Array[a, 60] (* _Jean-François Alcover_, Aug 28 2019 *)
%K A115123 easy,nonn
%O A115123 1,4
%A A115123 _Valery A. Liskovets_, Jan 17 2006
%E A115123 More terms from _Jean-François Alcover_, Aug 28 2019