a(n) = (2*n)!/(n!*a(n-1)), with a(0)=1.

Terms

a(0) =1a(1) =2a(2) =6a(3) =20a(4) =84a(5) =360a(6) =1848a(7) =9360a(8) =55440a(9) =318240a(10) =2106720a(11) =13366080a(12) =96909120a(13) =668304000a(14) =5233092480a(15) =38761632000a(16) =324451733760