a(n) = 7^n - n^9.

Terms

a(0) =1a(1) =6a(2) =-463a(3) =-19340a(4) =-259743a(5) =-1936318a(6) =-9960047a(7) =-39530064a(8) =-128452927a(9) =-347066882a(10) =-717524751a(11) =-380620948a(12) =8681506849a(13) =86284511034a(14) =657562026065