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%I #8 Apr 02 2019 16:42:54
%S 1,2,6,15,42,106,280,702,1778,4398,10910,26678,65172,157656,380524,
%T 912846,2185906,5216588,12433166,29564544,70189672,166245574,
%U 392909240,926290066,2178881218,5114469170,11985221654,28049398284,65588182636,153277006212,358073997608
%N Self-composition of the Euler totient function (A000010).
%H Vaclav Kotesovec, <a href="/A307308/b307308.txt">Table of n, a(n) for n = 1..500</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/TotientFunction.html">Totient Function</a>
%F 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 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 Cf. A000010, A008683, A010554, A065093, A307309.
%K nonn
%O 1,2
%A _Ilya Gutkovskiy_, Apr 02 2019
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