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 A007801 #28 Mar 20 2020 19:15:18
%S A007801 1,2,2,3,6,8,8,16,22,40,80,146,240,356,488,661,870,1184,1936,3750,
%T A007801 7976,17628,37528,74828,140480,249444,420392,678432,1056600,1593512,
%U A007801 2334928,3337410,4660246,6364080,8527984,11258182,14753032,19622940,27836440
%N A007801 Expansion of f(f(x)), where f = x + x^2 + x^4 + x^8 + x^16 + ...
%H A007801 Robert Israel, <a href="/A007801/b007801.txt">Table of n, a(n) for n = 1..1023</a>
%F A007801 a(n) = Sum_{k=1..n} A073266(n,k)*f(k). - _Vladimir Kruchinin_, Mar 13 2012
%p A007801 e:= 8;
%p A007801 f:= x -> sum(x^(2^i),i=0..e):
%p A007801 g:= f(f(x)):
%p A007801 S:= series(g,x,2^e):
%p A007801 seq(coeff(S,x,n),n=0..2^e-1); # _Robert Israel_, Aug 19 2014
%t A007801 f[x_]:= Sum[x^(2^j), {j,0,10}]; g[x_]:=f[f[x]]; CoefficientList[Series[g[x], {x, 0, 50}], x] (* _G. C. Greubel_, Mar 05 2020 *)
%K A007801 nonn
%O A007801 1,2
%A A007801 _Jeffrey Shallit_