A177481 Number of permutations of order n avoiding the consecutive pattern efgh with e

1, 1, 2, 6, 20, 80, 404, 2368, 15488, 114480, 948992, 8625672, 85223792, 913869056, 10567326528, 130796711016, 1726077013456, 24213357455936, 359694651093152, 5638959807231240, 93052021458248400, 1612444736747193696, 29271726199933801536, 555518182602741687432, 11001032351303890637648

Offset: 0

Author

Signy Olafsdottir (signy06(AT)ru.is), May 09 2010

Keywords

• nonn

Comments

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.

Links

• S. Kitaev, Introduction to partially ordered patterns, Discrete Applied Mathematics 155 (2007), 929-944.

Crossrefs

Cf. A117156, A177470, A177471, A177472, A177473, A177475, A177476, A177477, A177478, A177479, A177480, A177482, A177483, A177484, A376694.

Programs

Mathematica

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 *)

Extensions

a(0), a(10)-a(16) from Giovanni Resta, Mar 11 2020

Edited and a(17)-a(24) added by Max Alekseyev, Oct 01 2024