a(n) = (9*6^n+1)/5.

Terms

a(0) =2a(1) =11a(2) =65a(3) =389a(4) =2333a(5) =13997a(6) =83981a(7) =503885a(8) =3023309a(9) =18139853a(10) =108839117a(11) =653034701a(12) =3918208205a(13) =23509249229a(14) =141055495373a(15) =846332972237