(9^n - 5) / 4.

Terms

a(0) =1a(1) =19a(2) =181a(3) =1639a(4) =14761a(5) =132859a(6) =1195741a(7) =10761679a(8) =96855121a(9) =871696099a(10) =7845264901a(11) =70607384119a(12) =635466457081