G.f. = b(2)^2*b(4)/(2*x^5+x^4-2*x^3-x^2-x+1), where b(k) = (1-x^k)/(1-x).

Terms

a(0) =1a(1) =4a(2) =9a(3) =19a(4) =38a(5) =70a(6) =129a(7) =238a(8) =431a(9) =781a(10) =1419a(11) =2566a(12) =4640a(13) =8401a(14) =15192a(15) =27469a(16) =49691a(17) =89863a(18) =162498a(19) =293890a(20) =531485a(21) =961126a(22) =1738167a(23) =3143377a(24) =5684531a(25) =10280146a(26) =18591012a(27) =33620509a(28) =60800528a(29) =109953853