a(1)=4. For n > 1, a(n) = A129902(a(n-1))/2.

Terms

a(0) =4a(1) =6a(2) =12a(3) =30a(4) =60a(5) =180a(6) =630a(7) =1260a(8) =5040a(9) =27720a(10) =166320a(11) =1081080a(12) =8648640a(13) =73513440a(14) =698377680a(15) =8031343320a(16) =64250746560a(17) =803134332000