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 A355117 #5 Jun 24 2022 08:30:39
%S A355117 1,1,5,21,89,345,1405,5501,22033,87649,350405,1398981,5596345,
%T A355117 22373561,89492141,357930301,1431711857,5726679153,22906712645,
%U A355117 91626189381,366504720137,1466016390873,5864065352173,23456251396589,93825005578001,375299982311441,1501199928316661
%N A355117 a(1) = 1; a(n+1) = Sum_{d|n} 4^(n/d - 1) * a(d).
%F A355117 G.f.: x * ( 1 + Sum_{n>=1} a(n) * x^n / (1 - 4 * x^n) ).
%F A355117 a(n) ~ 4^(n-1) / 3.
%t A355117 a[1] = 1; a[n_] := a[n] = Sum[4^((n - 1)/d - 1) a[d], {d, Divisors[n - 1]}]; Table[a[n], {n, 1, 27}]
%Y A355117 Cf. A003238, A351405, A355116.
%K A355117 nonn
%O A355117 1,3
%A A355117 _Ilya Gutkovskiy_, Jun 19 2022