

A177476


Avoiding the pattern 231'1. To avoid 231'1 means not to have four consecutive letters such that if the third letter is removed, then in the obtained 3 letter word the smallest letter is the last one, and the largest letter is the second one.


3



1, 1, 2, 6, 20, 83, 402, 2245, 14192, 100650, 792508, 6859260, 64772648, 662630653, 7301841444, 86212535179, 1085834949064
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OFFSET

0,3


LINKS



MATHEMATICA

ok[{x_, y_, _, z_}] := Not[x>z && y>z && y>x]; a[n_] := Length@ Select[ Permutations@ Range@ n, AllTrue[ Partition[#, 4, 1], ok] &]; a /@ Range[0, 9]


CROSSREFS

Cf. A117156, A177470, A177471, A177472, A177473, A177474, A177475, A177477, A177478, A177479, A177480, A177481, A177482, A177483, A177484.


KEYWORD

nonn,more


AUTHOR

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


EXTENSIONS



STATUS

approved



