a(n) = n^7 + 7*n + 7^n.

Terms

a(0) =1a(1) =15a(2) =191a(3) =2551a(4) =18813a(5) =94967a(6) =397627a(7) =1647135a(8) =7862009a(9) =45136639a(10) =292475319a(11) =1996813991a(12) =13877119093a(13) =96951759015a(14) =678328486451