a(n) = 9*6^n - 1.

Terms

a(0) =8a(1) =53a(2) =323a(3) =1943a(4) =11663a(5) =69983a(6) =419903a(7) =2519423a(8) =15116543a(9) =90699263a(10) =544195583a(11) =3265173503a(12) =19591041023a(13) =117546246143a(14) =705277476863