a(n) = (9*8^n - 2)/7.

Terms

a(0) =1a(1) =10a(2) =82a(3) =658a(4) =5266a(5) =42130a(6) =337042a(7) =2696338a(8) =21570706a(9) =172565650a(10) =1380525202a(11) =11044201618a(12) =88353612946a(13) =706828903570