a(n) = 2*9^n-1.

Terms

a(0) =1a(1) =17a(2) =161a(3) =1457a(4) =13121a(5) =118097a(6) =1062881a(7) =9565937a(8) =86093441a(9) =774840977a(10) =6973568801a(11) =62762119217a(12) =564859072961