a(0) = 1; a(n) = Sum_{k=0..floor((n-1)/2)} (-3)^k * a(k) * a(n-2*k-1).

Terms

a(0) =1a(1) =1a(2) =1a(3) =-2a(4) =-5a(5) =1a(6) =16a(7) =94a(8) =127a(9) =-317a(10) =-1103a(11) =-2258a(12) =160a(13) =18922a(14) =39664a(15) =-163064a(16) =-524489a(17) =-187577a(18) =1711591a(19) =12353800a(20) =18926119a(21) =-53449661a(22) =-207918068a(23) =-15689834a(24) =960812728a(25) =3295064650a(26) =3870965368a(27) =-33248209286a(28) =-96830084768a(29) =17122290256