a(n) = (1/3!)*3^(n+1)*(n+5)*(n+1)*(n).

Terms

a(0) =18a(1) =189a(2) =1296a(3) =7290a(4) =36450a(5) =168399a(6) =734832a(7) =3070548a(8) =12400290a(9) =48715425a(10) =187067232a(11) =704690766a(12) =2611501074a(13) =9542023155a(14) =34437376800a(15) =122941435176a(16) =434685788658