a(n) = 7^n - n^12.

Terms

a(0) =1a(1) =6a(2) =-4047a(3) =-531098a(4) =-16774815a(5) =-244123818a(6) =-2176664687a(7) =-13840463658a(8) =-68713711935a(9) =-282389182874a(10) =-999717524751