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Search: id:A117106
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| A117106 |
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Number of permutations in S_n avoiding 21{bar 3}54 (i.e. every occurrence of 2154 is contained in an occurrence of a 21354). |
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+0
1
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| 1, 2, 6, 23, 104, 530, 2958, 17734, 112657, 750726, 5207910, 37387881, 276467208
(list; graph; listen)
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OFFSET
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1,2
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COMMENT
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The bar refers to a missing piece. In other words to say that a permutation has the pattern 21{bar 3}54 means that it has a 2154 (or equivalently a 2143) pattern but that there is no entry in the permutation so that we can extend this 2154 to a 21543 pattern.
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LINKS
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M. Bousquet-Melou and S. Butler, Forest-like permutations
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EXAMPLE
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a(4)=23 because the permutation 2143 has the pattern 21{bar 3}54, but none of the other 23 permutations in S_4 do.
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CROSSREFS
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Sequence in context: A005802 A061552 A053488 this_sequence A137534 A137535 A030266
Adjacent sequences: A117103 A117104 A117105 this_sequence A117107 A117108 A117109
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KEYWORD
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nonn
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AUTHOR
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Steve Butler (sbutler(AT)math.ucsd.edu), Apr 18 2006
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