a(n) = Sum_{k=0..n} binomial(3*n+k,n).

Terms

a(0) =1a(1) =7a(2) =64a(3) =589a(4) =5396a(5) =49259a(6) =448876a(7) =4088655a(8) =37259596a(9) =339893851a(10) =3104834668a(11) =28405204425a(12) =260285994164