a(n) = Sum_{k=0..n} (-1)^k * binomial(3*k,n-k) * Catalan(k).

Terms

a(0) =1a(1) =-1a(2) =-1a(3) =4a(4) =-2a(5) =-14a(6) =36a(7) =-1a(8) =-191a(9) =418a(10) =182a(11) =-2904a(12) =5546a(13) =5226a(14) =-47466a(15) =78843a(16) =121489a(17) =-813060a(18) =1157400a(19) =2659695a(20) =-14364997a(21) =17078752a(22) =56866292a(23) =-259091498a(24) =246553176a(25) =1202210486a(26) =-4736784794a(27) =3346401868a(28) =25255328948a(29) =-87323306720