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

Terms

a(0) =1a(1) =2a(2) =1a(3) =-1a(4) =1a(5) =8a(6) =1a(7) =-28a(8) =1a(9) =134a(10) =1a(11) =-649a(12) =1a(13) =3320a(14) =1a(15) =-17497a(16) =1a(17) =94526a(18) =1a(19) =-520507a(20) =1a(21) =2910896a(22) =1a(23) =-16487794a(24) =1a(25) =94393106a(26) =1a(27) =-545337199a(28) =1a(29) =3175320608