OFFSET
0,3
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.
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 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Signy Olafsdottir (signy06(AT)ru.is), May 09 2010
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
STATUS
approved