a(0) = 1; a(n) = Sum_{k=0..floor((n-1)/6)} a(6*k) * a(n-1-6*k).

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =1a(5) =1a(6) =1a(7) =2a(8) =3a(9) =4a(10) =5a(11) =6a(12) =7a(13) =15a(14) =24a(15) =34a(16) =45a(17) =57a(18) =70a(19) =154a(20) =253a(21) =368a(22) =500a(23) =650a(24) =819a(25) =1827a(26) =3045a(27) =4495a(28) =6200a(29) =8184