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

Terms

a(0) =1a(1) =6a(2) =45a(3) =378a(4) =3321a(5) =29646a(6) =266085a(7) =2392578a(8) =21526641a(9) =193720086a(10) =1743421725a(11) =15690618378a(12) =141215033961