a(n) = 5^n + 7^n + 8^n.

Terms

a(0) =3a(1) =20a(2) =138a(3) =980a(4) =7122a(5) =52700a(6) =395418a(7) =2998820a(8) =22932642a(9) =176524460a(10) =1365982698a(11) =10616089460a(12) =82804904562a(13) =647865527420