a(n) = A306898(n)/2.

Terms

a(0) =1a(1) =4a(2) =6a(3) =16a(4) =20a(5) =48a(6) =70a(7) =160a(8) =270a(9) =560a(10) =1034a(11) =2160a(12) =4108a(13) =8344a(14) =16440a(15) =33088a(16) =65552a(17) =131688a(18) =262162a(19) =525440a(20) =1048740a(21) =2099240a(22) =4194326a(23) =8393184a(24) =16777300a(25) =33562672a(26) =67109418a(27) =134234464a(28) =268435484a(29) =536904960