a(n) = T(n,[ n/2 ]+1), where T is the array defined in A025564.

Terms

a(0) =1a(1) =4a(2) =10a(3) =22a(4) =61a(5) =120a(6) =356a(7) =665a(8) =2058a(9) =3732a(10) =11892a(11) =21153a(12) =68860a(13) =120835a(14) =399828a(15) =694590a(16) =2328066a(17) =4013088a(18) =13591364a(19) =23284424a(20) =79538750a(21) =135580865a(22) =466489520a(23) =791862354a(24) =2741310614a(25) =4637009300a(26) =16137748980a(27) =27215510265a(28) =95152639095a(29) =160053426720