a(n) = 7^n - n^11.

Terms

a(0) =1a(1) =6a(2) =-1999a(3) =-176804a(4) =-4191903a(5) =-48811318a(6) =-362679407a(7) =-1976503200a(8) =-8584169791a(9) =-31340706002a(10) =-99717524751a(11) =-283334343868a(12) =-729167083487