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

A024488

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

Terms

    a(0) =3a(1) =18a(2) =210a(3) =3150a(4) =54054a(5) =1009008a(6) =19953648a(7) =411543990a(8) =8764362750a(9) =191413682460

External references