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 A177481 #19 Oct 02 2024 09:06:25
%S A177481 1,1,2,6,20,80,404,2368,15488,114480,948992,8625672,85223792,
%T A177481 913869056,10567326528,130796711016,1726077013456,24213357455936,
%U A177481 359694651093152,5638959807231240,93052021458248400,1612444736747193696,29271726199933801536,555518182602741687432,11001032351303890637648
%N A177481 Number of permutations of order n avoiding the consecutive pattern efgh with e<f, e<h, g<f and g<h.
%C A177481 To avoid efgh means not to have four consecutive letters such that the first and the third letters are less than the second and the fourth letters.
%H A177481 S. Kitaev, <a href="https://doi.org/10.1016/j.dam.2006.09.011">Introduction to partially ordered patterns</a>, Discrete Applied Mathematics 155 (2007), 929-944.
%t A177481 ok[{e_,f_,g_,h_}] := e>f || e>h || g>f || g>h; a[n_] := Length@ Select[ Permutations[ Range@ n], AllTrue[ Partition[#, 4, 1], ok] &]; Array[a, 9, 0] (* _Giovanni Resta_, Mar 11 2020 *)
%Y A177481 Cf. A117156, A177470, A177471, A177472, A177473, A177475, A177476, A177477, A177478, A177479, A177480, A177482, A177483, A177484, A376694.
%K A177481 nonn
%O A177481 0,3
%A A177481 Signy Olafsdottir (signy06(AT)ru.is), May 09 2010
%E A177481 a(0), a(10)-a(16) from _Giovanni Resta_, Mar 11 2020
%E A177481 Edited and a(17)-a(24) added by _Max Alekseyev_, Oct 01 2024