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 A121773 #15 May 22 2024 02:12:47
%S A121773 1,2,6,24,165,1560,19684,299600,5381685,111111340,2593758618,
%T A121773 67546215528,1941507500933,61054982558024,2085209014017960,
%U A121773 76861433640597376,3041324492665174641,128583032925805678368,5784852794346334629910,275941052631578947368440
%N A121773 Number of n-bead necklaces with n+1 colors for n>0, with a(0)=1.
%H A121773 Stefano Spezia, <a href="/A121773/b121773.txt">Table of n, a(n) for n = 0..350</a>
%F A121773 a(n) = (1/n)*Sum_{d|n} phi(n/d)*(n+1)^d, for n>0, with a(0)=1.
%F A121773 a(n) is divisible by (n+1); a(n)/(n+1) = A121774(n).
%t A121773 a[0] = 1; a[n_] := DivisorSum[n, (n+1)^# * EulerPhi[n/#] &] / n; Array[a, 20, 0] (* _Amiram Eldar_, Aug 15 2023 *)
%o A121773 (PARI) a(n)=if(n==0,1,(1/n)*sumdiv(n,d,eulerphi(n/d)*(n+1)^d))
%Y A121773 Cf. A121774, A056665.
%K A121773 nonn
%O A121773 0,2
%A A121773 _Paul D. Hanna_, Aug 20 2006
%E A121773 a(18)-a(19) from _Amiram Eldar_, Aug 15 2023