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 A024420 #20 Jul 13 2019 10:46:39
%S A024420 1,1,1,4,22,108,612,4416,36576,331200,3319200,36806400,445046400,
%T A024420 5813579520,81716947200,1230656716800,19761225523200,336973967769600,
%U A024420 6082189179494400,115851849523200000,2322322137354240000,48869666136023040000
%N A024420 a(n) = n! * Sum_{j=0..floor(n/2)} (-1)^j/binomial(n,j).
%p A024420 a:= n-> n!*add((-1)^j/binomial(n, j), j=0..iquo(n, 2)):
%p A024420 seq(a(n), n=0..23); # _Alois P. Heinz_, Jul 10 2019
%t A024420 Table[n!*Sum[(-1)^k/Binomial[n, k], {k, 0, Floor[n/2]}], {n, 0, 20}] (* _Vaclav Kotesovec_, Jul 10 2019 *)
%K A024420 nonn
%O A024420 0,4
%A A024420 _Clark Kimberling_
%E A024420 More terms from _Sean A. Irvine_, Jul 10 2019