a(n) = 4*(2n+1)!/n!^2.

Terms

a(0) =4a(1) =24a(2) =120a(3) =560a(4) =2520a(5) =11088a(6) =48048a(7) =205920a(8) =875160a(9) =3695120a(10) =15519504a(11) =64899744a(12) =270415600a(13) =1123264800a(14) =4653525600a(15) =19234572480a(16) =79342611480a(17) =326704870800