a(0) = ... = a(3) = 1; a(n) = Sum_{k=0..n-4} binomial(n-4,k) * a(k) * a(n-k-4).

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =1a(5) =2a(6) =4a(7) =8a(8) =16a(9) =34a(10) =82a(11) =226a(12) =678a(13) =2108a(14) =6892a(15) =23948a(16) =88532a(17) =344816a(18) =1401200a(19) =5925000a(20) =26146360a(21) =120743496a(22) =582606552a(23) =2926675112a(24) =15259183112a(25) =82458502624a(26) =461577781968