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A371657
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G.f. satisfies A(x) = 1 + x * A(x)^2 * (1 + A(x) + A(x)^2).
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2
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1, 3, 27, 333, 4752, 73764, 1209492, 20610693, 361403937, 6478386561, 118181952369, 2186908154748, 40949739595242, 774474351098031, 14772979729013247, 283878381945510621, 5490264493926636912, 106786725176131118523, 2087502569999563971843
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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) = (1/n) * Sum_{k=0..floor(n-1)/2} 3^(n-k) * binomial(n,k) * binomial(3*n-k,n-1-2*k) for n > 0.
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PROG
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(PARI) a(n) = if(n==0, 1, sum(k=0, (n-1)\2, 3^(n-k)*binomial(n, k)*binomial(3*n-k, n-1-2*k))/n);
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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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