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 A380020 #6 Jan 09 2025 08:00:20
%S A380020 1,2,8,55,540,6861,106828,1968443,41884496,1010558161,27259824996,
%T A380020 812935829355,26556802948624,943118750625377,36176486632451012,
%U A380020 1490585029223430691,65656827447552549504,3078782615385684631809,153127047650469476373316
%N A380020 Expansion of e.g.f. 1/sqrt(exp(-2*x) - 2*x*exp(-x)).
%F A380020 a(n) = n! * Sum_{k=0..n} (-2)^k * (k+1)^(n-k) * binomial(-1/2,k)/(n-k)!.
%o A380020 (PARI) a(n) = n!*sum(k=0, n, (-2)^k*(k+1)^(n-k)*binomial(-1/2, k)/(n-k)!);
%Y A380020 Cf. A380014, A380015.
%K A380020 nonn
%O A380020 0,2
%A A380020 _Seiichi Manyama_, Jan 09 2025