a(n) = 7^n-6.

Terms

a(0) =1a(1) =43a(2) =337a(3) =2395a(4) =16801a(5) =117643a(6) =823537a(7) =5764795a(8) =40353601a(9) =282475243a(10) =1977326737a(11) =13841287195a(12) =96889010401a(13) =678223072843