G.f.: A(x) = Sum_{n>=0} (3n)!/(n!)^3 * x^(3n)/(1-x-x^2)^(3n+1).

Terms

a(0) =1a(1) =1a(2) =2a(3) =9a(4) =29a(5) =92a(6) =343a(7) =1281a(8) =4720a(9) =17899a(10) =68933a(11) =266364a(12) =1037423a(13) =4072439a(14) =16065148a(15) =63658521a(16) =253356763a(17) =1012049086a(18) =4055596343a(19) =16299779331a(20) =65683233938a(21) =265310551667