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Search: id:A006335
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| A006335 |
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4^n*(3n)!/((n+1)!(2n+1)!).
(Formerly M2094)
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+0
6
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| 1, 2, 16, 192, 2816, 46592, 835584, 15876096, 315031552, 6466437120, 136383037440, 2941129850880, 64614360416256, 1442028424527872, 32619677465182208, 746569714888605696, 17262927525017812992
(list; graph; listen)
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OFFSET
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1,2
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COMMENT
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Number of planar lattice walks of length 3n starting and ending at (0,0), remaining in the first quadrant, and using only NE,W,S steps.
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REFERENCES
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G. Kreweras, Sur une classe de problemes de denombrement lies au treillis des partitions des entiers, Cahiers du Bureau Universitaire de Recherche Op\'{e}rationnelle, Institut de Statistique, Universit\'{e} de Paris, 6 (1965), circa p. 82.
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LINKS
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M. Bousquet-M\'elou, Walks in the quarter plane: Kreweras' algebraic model
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PROGRAM
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(PARI) a(n)=if(n<0, 0, 4^n*(3*n)!/(n+1)!/(2*n+1)!)
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CROSSREFS
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Equals 2^(n-1) * A000309(n-1) for n>1.
Cf. A098272. First row of array A098273.
Sequence in context: A011553 A123898 A118644 this_sequence A051711 A012683 A012677
Adjacent sequences: A006332 A006333 A006334 this_sequence A006336 A006337 A006338
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KEYWORD
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nonn,easy
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AUTHOR
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njas
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