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Search: id:A032351
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| A032351 |
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Number of permutations of length n which avoid the patterns 2143, 1324 (smooth permutations); or avoid the patterns 1342, 2431; etc. |
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+0
4
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| 1, 2, 6, 22, 88, 366, 1552, 6652, 28696, 124310, 540040, 2350820, 10248248, 44725516, 195354368, 853829272, 3733693872, 16333556838, 71476391800, 312865382004, 1369760107576, 5998008630244, 26268304208032
(list; graph; listen)
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OFFSET
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1,2
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REFERENCES
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R. P. Stanley, Enumerative Combinatorics, Cambridge, Vol. 2, 1999; see Problem 6.47.
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LINKS
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Miklos Bona, The permutation classes equinumerous to the smooth class, Electron. J. Combin., 5 (1998), no. 1, Research Paper 31, 12 pp.
M. Bousquet-Melou and S. Butler, Forest-like permutations
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FORMULA
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G.f.: A(x)=\frac{1-5x+3x^2+x^2\sqrt{1-4x}}{1-6^x+8x^2-4x^3}
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CROSSREFS
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Cf. A053617.
Sequence in context: A073075 A101043 A101046 this_sequence A109033 A049135 A049127
Adjacent sequences: A032348 A032349 A032350 this_sequence A032352 A032353 A032354
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KEYWORD
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nonn,easy,nice
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AUTHOR
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Miklos Bona (bona(AT)math.ufl.edu)
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EXTENSIONS
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More terms from Erich Friedman (erich.friedman(AT)stetson.edu).
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