a(n) = 6^n - n^7.

Terms

a(0) =1a(1) =5a(2) =-92a(3) =-1971a(4) =-15088a(5) =-70349a(6) =-233280a(7) =-543607a(8) =-417536a(9) =5294727a(10) =50466176a(11) =343309885a(12) =2140950528a(13) =12997945499a(14) =78258750592a(15) =470014125201