a(n) = 4^n + 5^n + 7^n.

Terms

a(0) =3a(1) =16a(2) =90a(3) =532a(4) =3282a(5) =20956a(6) =137370a(7) =918052a(8) =6220962a(9) =42568876a(10) =293289450a(11) =2030349172a(12) =14102205042a(13) =98176822396a(14) =684595023930