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A024486
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a(n) = (1/(2n+1))*Multinomial(3n; n,n,n).
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1
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1, 2, 18, 240, 3850, 68796, 1319472, 26604864, 556794810, 11993338500, 264333180540, 5935956324480, 135389270500848, 3128818447143200, 73123195376232000, 1725644509205633280, 41071074678970154010, 984853105623747983700, 23773506148264947574500
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OFFSET
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0,2
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LINKS
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FORMULA
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a(n) = (3*n)! / ((2*n+1) * (n!)^3).
D-finite with recurrence: n^2*(2*n+1)*a(n) = 3*(2*n-1)*(3*n-2)*(3*n-1)*a(n-1).
a(n) ~ 3^(3*n+1/2) / (4*Pi*n^2).
(End)
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MATHEMATICA
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Table[Multinomial[n, n, n]/(2*n+1), {n, 0, 20}] (* Vaclav Kotesovec, Feb 19 2015 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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