Duplicate of A024537.

Terms

a(0) =2a(1) =4a(2) =9a(3) =21a(4) =50a(5) =120a(6) =289a(7) =697a(8) =1682a(9) =4060a(10) =9801a(11) =23661a(12) =57122a(13) =137904a(14) =332929