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A005560
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Number of walks on square lattice.
(Formerly M2987)
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2
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1, 3, 15, 45, 189, 588, 2352, 7560, 29700, 98010, 382239, 1288287, 5010005, 17177160, 66745536, 232092432, 901995588, 3173688180, 12342120700, 43861998180
(list; graph; refs; listen; history; internal format)
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OFFSET
| 2,2
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REFERENCES
| N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
| R. K. Guy, Catwalks, Sandsteps and Pascal Pyramids, J. Integer Seqs., Vol. 3 (2000), #00.1.6
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FORMULA
| a(n) = C(n+3, ceil(n/2))*C(n+2, floor(n/2)) - C(n+3, ceil((n-1)/2))*C(n+2, floor((n-1)/2)). - Paul D. Hanna (pauldhanna(AT)juno.com), Apr 16 2004
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PROG
| (PARI) {a(n)=binomial(n+3, ceil(n/2))*binomial(n+2, floor(n/2)) - binomial(n+3, ceil((n-1)/2))*binomial(n+2, floor((n-1)/2))}
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CROSSREFS
| Cf. A005558-A005562, A093768.
Sequence in context: A030505 A074355 A201868 * A100747 A100737 A178669
Adjacent sequences: A005557 A005558 A005559 * A005561 A005562 A005563
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KEYWORD
| nonn
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AUTHOR
| N. J. A. Sloane (njas(AT)research.att.com).
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