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A177480
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Suppose e<f, e<h, g<f and g<h. To avoid egfh means not to have four consecutive letters such that the first and the second letters are less than the third and the fourth letters.
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2
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1, 1, 2, 6, 20, 84, 412, 2300, 14676, 104536, 825660, 7168860, 67826340, 695174208, 7671602644, 90700227700, 1143825611348
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OFFSET
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0,3
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LINKS
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MATHEMATICA
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ok[{e_, f_, g_, h_}] := e > g || e > h || f > g || f > h; a[n_] := Length@ Select[ Permutations[Range@n], AllTrue[ Partition[#, 4, 1], ok] &]; a /@ Range[0, 9] (* Giovanni Resta, Mar 11 2020 *)
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CROSSREFS
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Cf. A117156, A177470, A177471, A177472, A177473, A177474, A177475, A177476, A177477, A177478, A177479, A177481, A177482, A177483, A177484.
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KEYWORD
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nonn,more
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AUTHOR
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Signy Olafsdottir (signy06(AT)ru.is), May 09 2010
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EXTENSIONS
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STATUS
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approved
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