a(n) = (2*n)!*(2*n+1)! / n!^2.A000909a(n) = (2*n)!*(2*n+1)! / n!^2.Terms a(0) =1a(1) =12a(2) =720a(3) =100800a(4) =25401600a(5) =10059033600External referencesoeis: A000909