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 A121647 #12 Dec 11 2019 05:05:09 %S A121647 1,3,2,6,4,3,9,2,8,6,7,4,5,12,7,3,4,10,9,7,2,5,8,6,5,14,4,10,7,5,7,5, %T A121647 12,10,4,3,9,14,3,10,8,10,11,15,4,9,4,2,9,3,5,6,4,7,2,8,5,15,4,11,4,5, %U A121647 10,16,7,10,6,13,9,2,6,7,5,4,4,6,2,8,2,15,5,8,4,11,10,12,8 %N A121647 Sequence gives values of sigma(n)^2 divided by UnitarySigma(n)*UnitaryPhi(n) associated with n in A121556. %H A121647 Amiram Eldar, <a href="/A121647/b121647.txt">Table of n, a(n) for n = 1..1000</a> %F A121647 a(n) = f(A121556(n)), where f(m) = A000203(m)^2/(A034448(m)*A047994(m)). - _Amiram Eldar_, Dec 11 2019 %t A121647 f[p_, e_] := (p^(e+1)-1)^2/(p-1)^2/(p^(2*e)-1); ratio[1] = 1; ratio[n_] := Times @@ (f @@@ FactorInteger[n]); Select[ratio /@ Range[10^5], IntegerQ] (* _Amiram Eldar_, Dec 11 2019 *) %Y A121647 Cf. A000203, A034448, A047994, A121556. %K A121647 nonn %O A121647 1,2 %A A121647 _Yasutoshi Kohmoto_ Sep 12 2006 %E A121647 Extended with the aid of b121556.txt. - _R. J. Mathar_, Sep 04 2018