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A370218
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Coefficient of x^n in the expansion of ( (1+x)^3 / (1-x^3)^3 )^n.
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1
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1, 3, 15, 93, 639, 4578, 33423, 246816, 1838367, 13788399, 104011140, 788315124, 5998380543, 45794787678, 350619595614, 2691082393818, 20699166876831, 159515321712480, 1231354153215123, 9519556856284218, 73694926944160164, 571201080979318470
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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) = Sum_{k=0..floor(n/3)} binomial(3*n+k-1,k) * binomial(3*n,n-3*k).
The g.f. exp( Sum_{k>=1} a(k) * x^k/k ) has integer coefficients and equals (1/x) * Series_Reversion( x / (1+x)^3 * (1-x^3)^3 ). See A369403.
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PROG
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(PARI) a(n, s=3, t=3, u=3) = sum(k=0, n\s, binomial(t*n+k-1, k)*binomial(u*n, n-s*k));
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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