a(n) = (2*6^(n+1) - 7) / 5.

Terms

a(0) =1a(1) =13a(2) =85a(3) =517a(4) =3109a(5) =18661a(6) =111973a(7) =671845a(8) =4031077a(9) =24186469a(10) =145118821a(11) =870712933a(12) =5224277605a(13) =31345665637a(14) =188073993829