a(n) = (1/6)*multinomial(3*n;n,n,n).

A060542

a(n) = (1/6)*multinomial(3*n;n,n,n).

Terms

    a(0) =1a(1) =15a(2) =280a(3) =5775a(4) =126126a(5) =2858856a(6) =66512160a(7) =1577585295a(8) =37978905250a(9) =925166131890

External references