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

Terms

a(0) =3a(1) =1a(2) =1a(3) =3a(4) =9a(5) =27a(6) =84a(7) =261a(8) =810a(9) =2514a(10) =8046a(11) =25704a(12) =81975a(13) =261261a(14) =854037a(15) =2784540a(16) =9057393a(17) =29439855a(18) =97650930a(19) =323099676a(20) =1066565907a(21) =3517004046a(22) =11791407663a(23) =39431809098a(24) =131550859650a(25) =438453827928