A379185 G.f. A(x) satisfies A(x) = 1/((1 - x*A(x)^2) * (1 - x*A(x))^2).
1, 3, 18, 139, 1222, 11618, 116372, 1209779, 12930966, 141225530, 1569136588, 17680779230, 201562070356, 2320574126216, 26942875644408, 315109464849603, 3708926665685286, 43901133108206978, 522240410257549260, 6240258006163094026, 74864641626913850964
Offset: 0
Keywords
Programs
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PARI
a(n) = sum(k=0, n, binomial(n+2*k+1, k)*binomial(3*n+k+1, n-k)/(n+2*k+1));
Formula
a(n) = Sum_{k=0..n} binomial(n+2*k+1,k) * binomial(3*n+k+1,n-k)/(n+2*k+1).