2^(n+1)*P_n(1/2) (see A155100).

Terms

a(0) =1a(1) =1a(2) =5a(3) =10a(4) =70a(5) =440a(6) =4280a(7) =44560a(8) =575920a(9) =8224640a(10) =134583680a(11) =2422639360a(12) =48232203520