G.f.: 1/[(1-2x)(1+2x+3x^2)].

Terms

a(0) =1a(1) =0a(2) =1a(3) =6a(4) =1a(5) =12a(6) =37a(7) =18a(8) =109a(9) =240a(10) =217a(11) =894a(12) =1657a(13) =2196a(14) =7021a(15) =12138a(16) =20197a(17) =54264a(18) =93025a(19) =175446a(20) =418609a(21) =733596a(22) =1471285a(23) =3245250a(24) =5872861a(25) =12072960a(26) =25344361a(27) =47310126a(28) =97782121a(29) =199376292