a(n) = 4^n + 7^n + 8^n.

Terms

a(0) =3a(1) =19a(2) =129a(3) =919a(4) =6753a(5) =50599a(6) =383889a(7) =2937079a(8) =22607553a(9) =174833479a(10) =1357265649a(11) =10571455639a(12) =82577541153a(13) =646711933159