a(n) = 7^n-2^n+1.

Terms

a(0) =1a(1) =6a(2) =46a(3) =336a(4) =2386a(5) =16776a(6) =117586a(7) =823416a(8) =5764546a(9) =40353096a(10) =282474226a(11) =1977324696a(12) =13841283106a(13) =96889002216a(14) =678223056466