a(n) = (8^n - 1)/7.

Terms

a(0) =0a(1) =1a(2) =9a(3) =73a(4) =585a(5) =4681a(6) =37449a(7) =299593a(8) =2396745a(9) =19173961a(10) =153391689a(11) =1227133513a(12) =9817068105a(13) =78536544841a(14) =628292358729