a(n) = ((2*n + 1)!! + 1)/2.

Terms

a(0) =1a(1) =2a(2) =8a(3) =53a(4) =473a(5) =5198a(6) =67568a(7) =1013513a(8) =17229713a(9) =327364538a(10) =6874655288a(11) =158117071613