a(n) = 2^n + 7^n + 9^n.

Terms

a(0) =3a(1) =18a(2) =134a(3) =1080a(4) =8978a(5) =75888a(6) =649154a(7) =5606640a(8) =48811778a(9) =427774608a(10) =3769260674a(11) =33358388400a(12) =296270827778