A379159 G.f. A(x) satisfies A(x) = (1 + x^3) * (1 + x*A(x)^3).
1, 1, 3, 13, 59, 294, 1548, 8473, 47694, 274347, 1605553, 9529080, 57219636, 346989180, 2121996165, 13071868062, 81039237719, 505230120558, 3165528142389, 19922166970041, 125882558097870, 798300369383649, 5079196380442687, 32413550247722622, 207420591733202421
Offset: 0
Keywords
Programs
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PARI
a(n) = sum(k=0, n\3, binomial(3*n-9*k+1, k)*binomial(3*n-9*k+1, n-3*k)/(3*n-9*k+1));
Formula
a(n) = Sum_{k=0..floor(n/3)} binomial(3*n-9*k+1,k) * binomial(3*n-9*k+1,n-3*k)/(3*n-9*k+1).