a(n) = 3^n*(3^n-1)/2.

Terms

a(0) =0a(1) =3a(2) =36a(3) =351a(4) =3240a(5) =29403a(6) =265356a(7) =2390391a(8) =21520080a(9) =193700403a(10) =1743362676a(11) =15690441231a(12) =141214502520