A379173 G.f. A(x) satisfies A(x) = (1 + x)/(1 - x*A(x))^2.
1, 3, 11, 53, 284, 1630, 9794, 60830, 387390, 2515892, 16599051, 110943779, 749603067, 5111606801, 35133394554, 243146923574, 1692918638012, 11850006727400, 83341778073920, 588646472669454, 4173607638548291, 29694593381322531, 211941668053441490, 1517087043428034420
Offset: 0
Keywords
Programs
-
PARI
a(n) = sum(k=0, n, binomial(n-k+1, k)*binomial(3*n-3*k+1, n-k)/(n-k+1));
Formula
a(n) = Sum_{k=0..n} binomial(n-k+1,k) * binomial(3*n-3*k+1,n-k)/(n-k+1).