G.f. = b(2)*b(4)*b(6)/(x^9+x^8+x^7-2*x^3-x^2-x+1), where b(k) = (1-x^k)/(1-x).

Terms

a(0) =1a(1) =4a(2) =10a(3) =23a(4) =49a(5) =100a(6) =202a(7) =404a(8) =804a(9) =1598a(10) =3173a(11) =6297a(12) =12494a(13) =24786a(14) =49168a(15) =97532a(16) =193466a(17) =383759a(18) =761221a(19) =1509948a(20) =2995110a(21) =5941052a(22) =11784572a(23) =23375678a(24) =46367597a(25) =91973973a(26) =182437998a(27) =361880886a(28) =717820720a(29) =1423856868