G.f.: exp( Sum_{n>=1} (x^n/n)*Sum_{k=0..n} C(3n,3k)*x^k ).

Terms

a(0) =1a(1) =1a(2) =2a(3) =12a(4) =41a(5) =134a(6) =535a(7) =2100a(8) =8008a(9) =31483a(10) =125405a(11) =498762a(12) =1995254a(13) =8032580a(14) =32433136a(15) =131333532a(16) =533489683a(17) =2172483451a(18) =8865550544a(19) =36251329866a(20) =148498309280a(21) =609271648415