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 A121556 #13 Dec 11 2019 05:05:05
%S A121556 1,2,3,6,14,15,30,35,42,70,78,105,190,210,348,357,418,570,714,910,
%T A121556 1045,1144,1254,2090,2296,2730,3135,3432,4060,4522,4674,5278,6270,
%U A121556 6888,10168,10659,10824,12180,12441,13566,14630,15834,16770,17160
%N A121556 Numbers such that sigma(n)^2 is divisible by UnitarySigma(n)*UnitaryPhi(n).
%H A121556 Donovan Johnson, <a href="/A121556/b121556.txt">Table of n, a(n) for n = 1..1000</a>
%p A121556 for n from 1 to 20000 do if numtheory[sigma](n)^2 mod (A047994(n)*A034448(n)) = 0 then printf("%d,",n) ; end if;end do:
%t A121556 f[p_, e_] := (p^(e+1)-1)^2/(p-1)^2/(p^(2*e)-1); seqQ[1] = True; seqQ[n_] := IntegerQ [Times @@ (f @@@ FactorInteger[n])]; Select[Range[17160], seqQ] (* _Amiram Eldar_, Dec 11 2019 *)
%Y A121556 Cf. A000203, A121647, A078557, A020492, A034448, A047994.
%K A121556 nonn
%O A121556 1,2
%A A121556 _Yasutoshi Kohmoto_, Sep 12 2006