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

Terms

a(0) =1a(1) =0a(2) =0a(3) =1a(4) =1a(5) =0a(6) =1a(7) =20a(8) =1a(9) =1a(10) =84a(11) =84a(12) =2a(13) =220a(14) =924a(15) =221a(16) =456a(17) =5005a(18) =5006a(19) =1271a(20) =18565a(21) =48621a(22) =19894a(23) =55080a(24) =293932a(25) =295954a(26) =188860a(27) =1308835a(28) =2707082a(29) =1603514