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A201168
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The number of permutations avoiding the "boxed" pattern 123.
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0
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1, 1, 2, 5, 15, 51, 194, 810, 3675, 17935, 93481, 517129, 3021133
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OFFSET
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0,3
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COMMENTS
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The statement in the Avgustinovich, Kitaev and Valyuzhenich paper that a(6) is greater than 303 is easily seen to be wrong, since that would require (among other constraints) that no more than 2 boxed-123-avoiding permutations of length 5 end in an ascent. - Peter J. Taylor, Apr 27 2015
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LINKS
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Table of n, a(n) for n=0..12.
Sergey Avgustinovich, Sergey Kitaev and Alexander Valyuzhenich, Avoidance of boxed mesh patterns on permutations.
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MATHEMATICA
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valid[l_] := valid[l] = Which[Length@l<3, True, Length@l==3, !Less@@l, True, valid[Most@l]&&valid[Rest@l]&&valid[DeleteCases[l, Min@l]]&&valid[DeleteCases[l, Max@l]]]; Length@Select[Permutations@Range@#, valid] & /@ Range[0, 9]
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CROSSREFS
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Sequence in context: A304454 A287253 A117426 * A001681 A343665 A192553
Adjacent sequences: A201165 A201166 A201167 * A201169 A201170 A201171
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KEYWORD
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nonn,more
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AUTHOR
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N. J. A. Sloane, May 06 2012
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EXTENSIONS
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More terms and Mathematica program from Peter J. Taylor, Apr 27 2015
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STATUS
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approved
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