a(n) = (11*6^n - 1)/5.

Terms

a(0) =2a(1) =13a(2) =79a(3) =475a(4) =2851a(5) =17107a(6) =102643a(7) =615859a(8) =3695155a(9) =22170931a(10) =133025587a(11) =798153523a(12) =4788921139a(13) =28733526835a(14) =172401161011