a(n) = (1/(9n-3))*M(3n; n,n,n), where M() is a multinomial coefficient.

A024489

a(n) = (1/(9n-3))*M(3n; n,n,n), where M() is a multinomial coefficient.

Terms

    a(0) =1a(1) =6a(2) =70a(3) =1050a(4) =18018a(5) =336336a(6) =6651216a(7) =137181330a(8) =2921454250a(9) =63804560820

External references