a(n) = Sum_{k=0..floor(n/2)} binomial(3*k,3*(n-2*k)).

Terms

a(0) =1a(1) =0a(2) =1a(3) =1a(4) =1a(5) =20a(6) =2a(7) =84a(8) =85a(9) =221a(10) =925a(11) =675a(12) =5007a(13) =5821a(14) =19020a(15) =49951a(16) =72829a(17) =296770a(18) =428528a(19) =1364693a(20) =3001497a(21) =5998390a(22) =18112232a(23) =31698490a(24) =92287620a(25) =193987770a(26) =443261263a(27) =1145617890a(28) =2292692419a(29) =6134650552