a(n) = 10^n + 6^n - 1.

Terms

a(0) =1a(1) =15a(2) =135a(3) =1215a(4) =11295a(5) =107775a(6) =1046655a(7) =10279935a(8) =101679615a(9) =1010077695a(10) =10060466175a(11) =100362797055