Duplicate of A006253.

Terms

a(0) =1a(1) =2a(2) =9a(3) =32a(4) =121a(5) =450a(6) =1681a(7) =6272a(8) =23409a(9) =87362a(10) =326041a(11) =1216800a(12) =4541161a(13) =16947842