a(0) = 1; a(n) = Sum_{k=0..floor((n-1)/2)} (-4)^k * a(k) * a(n-2*k-1).

Terms

a(0) =1a(1) =1a(2) =1a(3) =-3a(4) =-7a(5) =5a(6) =33a(7) =269a(8) =393a(9) =-1451a(10) =-4815a(11) =-14115a(12) =-2791a(13) =171685a(14) =398145a(15) =-3887699a(16) =-10399319a(17) =6567925a(18) =63031889a(19) =558518141a(20) =853157689a(21) =-4400392635a(22) =-14954126751a(23) =29904043597a(24) =151457170889a(25) =344861133205a(26) =170895616881