a(n) = (3*9^n - 1)/2.

Terms

a(0) =1a(1) =13a(2) =121a(3) =1093a(4) =9841a(5) =88573a(6) =797161a(7) =7174453a(8) =64570081a(9) =581130733a(10) =5230176601a(11) =47071589413a(12) =423644304721