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

Terms

a(0) =1a(1) =0a(2) =6a(3) =30a(4) =186a(5) =1140a(6) =7116a(7) =44856a(8) =285066a(9) =1823232a(10) =11721726a(11) =75683718a(12) =490429224a(13) =3187723344a(14) =20774505408a(15) =135699314640a(16) =888177411018