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

Terms

a(0) =2a(1) =14a(2) =122a(3) =1094a(4) =9842a(5) =88574a(6) =797162a(7) =7174454a(8) =64570082a(9) =581130734a(10) =5230176602a(11) =47071589414a(12) =423644304722