a(n) = (2/3)*(2*n-1)!*binomial(3*n,2*n).

A082787

a(n) = (2/3)*(2*n-1)!*binomial(3*n,2*n).

Terms

    a(0) =2a(1) =60a(2) =6720a(3) =1663200a(4) =726485760a(5) =494010316800

External references