a(n) = Sum_{i=1..floor((n+2)/4)} a(2i-1)*a(n-2i+1), with a(1)=a(3)=2 and a(2)=1.

Terms

a(0) =2a(1) =1a(2) =2a(3) =4a(4) =8a(5) =20a(6) =48a(7) =112a(8) =264a(9) =688a(10) =1760a(11) =4432a(12) =11136a(13) =30208a(14) =80160a(15) =209344a(16) =547584a(17) =1498752a(18) =4052224a(19) =10840064a(20) =28972416a(21) =80315136a(22) =220124160a(23) =597656576a(24) =1623036928a(25) =4522262528a(26) =12503194624a(27) =34330329088a(28) =94172397568a(29) =264205592576