a(n) = 3^n + 4^n.

Terms

a(0) =2a(1) =7a(2) =25a(3) =91a(4) =337a(5) =1267a(6) =4825a(7) =18571a(8) =72097a(9) =281827a(10) =1107625a(11) =4371451a(12) =17308657a(13) =68703187a(14) =273218425a(15) =1088090731a(16) =4338014017a(17) =17309009347a(18) =69106897225a(19) =276040168411