G.f.: 2*(1-x^3)/((1-x)^5*(1+x)^2).

Terms

a(0) =2a(1) =6a(2) =16a(3) =30a(4) =54a(5) =84a(6) =128a(7) =180a(8) =250a(9) =330a(10) =432a(11) =546a(12) =686a(13) =840a(14) =1024a(15) =1224a(16) =1458a(17) =1710a(18) =2000a(19) =2310a(20) =2662a(21) =3036a(22) =3456a(23) =3900a(24) =4394a(25) =4914a(26) =5488a(27) =6090a(28) =6750a(29) =7440