a(n) = Sum_{k=0..n} binomial(3*n-2,k).

Terms

a(0) =1a(1) =2a(2) =11a(3) =64a(4) =386a(5) =2380a(6) =14893a(7) =94184a(8) =600370a(9) =3850756a(10) =24821333a(11) =160645504a(12) =1043243132a(13) =6794414896a(14) =44360053772a(15) =290244832992