a(n) = 2^n + 4^n + 8^n.

Terms

a(0) =3a(1) =14a(2) =84a(3) =584a(4) =4368a(5) =33824a(6) =266304a(7) =2113664a(8) =16843008a(9) =134480384a(10) =1074791424a(11) =8594130944a(12) =68736258048a(13) =549822930944