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

Terms

a(0) =1a(1) =6a(2) =-79a(3) =-1844a(4) =-13983a(5) =-61318a(6) =-162287a(7) =0a(8) =3667649a(9) =35570638a(10) =272475249a(11) =1957839572a(12) =13805455393a(13) =96826261890a(14) =678117659345