a(n) = (3*n+1)!/((2*n)! * n!).

Terms

a(0) =1a(1) =12a(2) =105a(3) =840a(4) =6435a(5) =48048a(6) =352716a(7) =2558160a(8) =18386775a(9) =131231100a(10) =931395465a(11) =6580248480a(12) =46312074900a(13) =324897017760