a(n) = 7^n - 2^n.

Terms

a(0) =0a(1) =5a(2) =45a(3) =335a(4) =2385a(5) =16775a(6) =117585a(7) =823415a(8) =5764545a(9) =40353095a(10) =282474225a(11) =1977324695a(12) =13841283105a(13) =96889002215a(14) =678223056465