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A372372
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Coefficient of x^n in the expansion of ( (1+x+x^3)^3 / (1+x) )^n.
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2
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1, 2, 6, 29, 154, 792, 4047, 20967, 110058, 582257, 3096516, 16539690, 88684291, 477080671, 2573652045, 13917256254, 75417513498, 409447753305, 2226602481387, 12126317466294, 66129124080804, 361059693982863, 1973513103986606, 10797777359289435
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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) * binomial(2*n-k,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) / (1+x+x^3)^3 ). See A372376.
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PROG
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(PARI) a(n, s=3, t=3, u=-1) = sum(k=0, n\s, binomial(t*n, k)*binomial((t+u)*n-k, 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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