P_n(2) (see A155100).

Terms

a(0) =1a(1) =2a(2) =5a(3) =20a(4) =130a(5) =1120a(6) =12080a(7) =156320a(8) =2360080a(9) =40721920a(10) =790465280a(11) =17048837120a(12) =404482209280