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

Terms

a(0) =2a(1) =2a(2) =6a(3) =30a(4) =210a(5) =1890a(6) =20790a(7) =270270a(8) =4054050a(9) =68918850a(10) =1309458150a(11) =27498621150a(12) =632468286450