a(n) = 1^n + 2^n + 7^n.

Terms

a(0) =3a(1) =10a(2) =54a(3) =352a(4) =2418a(5) =16840a(6) =117714a(7) =823672a(8) =5765058a(9) =40354120a(10) =282476274a(11) =1977328792a(12) =13841291298a(13) =96889018600a(14) =678223089234