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A370097
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a(n) = Sum_{k=0..n} binomial(3*n,k) * binomial(3*n-k-1,n-k).
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1
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1, 5, 49, 545, 6401, 77505, 956929, 11976193, 151388161, 1928363009, 24712450049, 318255628289, 4115300220929, 53396370030593, 694845537386497, 9064787191660545, 118516719269445633, 1552528215946035201, 20372392543502991361, 267736366910401413121
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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) = [x^n] ( (1+x)^3/(1-x)^2 )^n.
The g.f. exp( Sum_{k>=1} a(k) * x^k/k ) has integer coefficients and equals (1/x) * Series_Reversion( x*(1-x)^2/(1+x)^3 ). See A365842.
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PROG
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(PARI) a(n) = sum(k=0, n, binomial(3*n, k)*binomial(3*n-k-1, n-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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