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 A307308 #9 Feb 16 2025 08:33:55
%S A307308 1,2,6,15,42,106,280,702,1778,4398,10910,26678,65172,157656,380524,
%T A307308 912846,2185906,5216588,12433166,29564544,70189672,166245574,
%U A307308 392909240,926290066,2178881218,5114469170,11985221654,28049398284,65588182636,153277006212,358073997608
%N A307308 Self-composition of the Euler totient function (A000010).
%H A307308 Vaclav Kotesovec, <a href="/A307308/b307308.txt">Table of n, a(n) for n = 1..500</a>
%H A307308 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/TotientFunction.html">Totient Function</a>
%F A307308 G.f.: g(g(x)), where g(x) = Sum_{k>=1} mu(k)*x^k/(1 - x^k)^2 is the g.f. of A000010.
%t A307308 g[x_] := g[x] = Sum[MoebiusMu[k] x^k/(1 - x^k)^2, {k, 1, 31}]; a[n_] := a[n] = SeriesCoefficient[g[g[x]], {x, 0, n}]; Table[a[n], {n, 31}]
%Y A307308 Cf. A000010, A008683, A010554, A065093, A307309.
%K A307308 nonn
%O A307308 1,2
%A A307308 _Ilya Gutkovskiy_, Apr 02 2019