a(n) = 3^n + 7^n + 9^n.

Terms

a(0) =3a(1) =19a(2) =139a(3) =1099a(4) =9043a(5) =76099a(6) =649819a(7) =5608699a(8) =48818083a(9) =427793779a(10) =3769318699a(11) =33358563499a(12) =296271355123