a(n) = 7^n + 10^n.

Terms

a(0) =2a(1) =17a(2) =149a(3) =1343a(4) =12401a(5) =116807a(6) =1117649a(7) =10823543a(8) =105764801a(9) =1040353607a(10) =10282475249a(11) =101977326743