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 A330506 #4 Dec 16 2019 14:25:43
%S A330506 1,2,7,24,129,840,5265,40320,434385,3900960,40809825,558835200,
%T A330506 6335076825,91070179200,1641957141825,20922789888000,359796258446625,
%U A330506 7663952552256000,122832552380162625,2615369658789888000,62315614994643635625
%N A330506 Expansion of e.g.f. Sum_{k>=1} arcsin(x^k).
%F A330506 a(n) = n! * Sum_{d|n, d odd} ((d - 2)!!)^2 / d!.
%t A330506 nmax = 21; CoefficientList[Series[Sum[ArcSin[x^k], {k, 1, nmax}], {x, 0, nmax}], x] Range[0, nmax]! // Rest
%t A330506 Table[n! DivisorSum[n, ((# - 2)!!)^2/#! &, OddQ[#] &], {n, 1, 21}]
%Y A330506 Cf. A001818, A176473, A184877, A330254, A330505.
%K A330506 nonn
%O A330506 1,2
%A A330506 _Ilya Gutkovskiy_, Dec 16 2019