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A370271
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Coefficient of x^n in the expansion of 1/( (1-x)^3 * (1-x^2)^3 )^n.
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1
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1, 3, 27, 246, 2379, 23628, 239058, 2450052, 25351755, 264270765, 2771024652, 29194911342, 308813298690, 3277454178144, 34883317836240, 372195546176496, 3979793738688075, 42635773396647054, 457529396858568837, 4917191231017846902, 52917857164300253004
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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/2)} binomial(3*n+k-1,k) * binomial(4*n-2*k-1,n-2*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^2)^3 ). See A368079.
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PROG
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(PARI) a(n, s=2, t=3, u=3) = sum(k=0, n\s, binomial(t*n+k-1, k)*binomial((u+1)*n-s*k-1, 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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