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

Terms

a(0) =1a(1) =2a(2) =1a(3) =-5a(4) =1a(5) =89a(6) =1a(7) =-2603a(8) =1a(9) =133265a(10) =1a(11) =-10449779a(12) =1a(13) =1161734969a(14) =1a(15) =-173838018059a(16) =1a(18) =1a(20) =1a(22) =1a(24) =1a(26) =1a(28) =1