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

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =5a(5) =9a(6) =17a(7) =35a(8) =65a(9) =125a(10) =241a(11) =463a(12) =885a(13) =1701a(14) =3265a(15) =6271a(16) =12033a(17) =23105a(18) =44353a(19) =85147a(20) =163445a(21) =313777a(22) =602353a(23) =1156339a(24) =2219809a(25) =4261389a(26) =8180561a(27) =15704215a(28) =30147333a(29) =57873821