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

Terms

a(0) =1a(1) =2a(2) =12a(3) =100a(4) =980a(5) =10584a(6) =121968a(7) =1472328a(8) =18404100a(9) =236390440a(10) =3103161776a(11) =41469525552a(12) =562496897872