a(n) = 3^(n-1) * n! * Catalan(n-1).

A221954

a(n) = 3^(n-1) * n! * Catalan(n-1).

Terms

    a(0) =1a(1) =6a(2) =108a(3) =3240a(4) =136080a(5) =7348320a(6) =484989120a(7) =37829151360

External references