A371989 G.f. A(x) satisfies A(x) = ( 1 + 9*x*A(x)*(1 + A(x)) )^(1/3).
1, 6, 18, -18, -270, 486, 7938, -17010, -290142, 676998, 11852082, -29091474, -518183406, 1315388646, 23720180130, -61672468338, -1122411776958, 2971089997830, 54459919690578, -146181889695762, -2694816626063310, 7314502270743270, 135469313469987138
Offset: 0
Keywords
Programs
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PARI
a(n) = 9^n*sum(k=0, n, binomial(n, k)*binomial(n/3+k/3+1/3, n)/(n+k+1));
Formula
a(n) = 9^n * Sum_{k=0..n} binomial(n,k) * binomial(n/3+k/3+1/3,n)/(n+k+1).