a(n) = 7^n - 6^n.

Terms

a(0) =0a(1) =1a(2) =13a(3) =127a(4) =1105a(5) =9031a(6) =70993a(7) =543607a(8) =4085185a(9) =30275911a(10) =222009073a(11) =1614529687a(12) =11664504865a(13) =83828316391a(14) =599858908753