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

Terms

a(0) =2a(1) =12a(2) =80a(3) =576a(4) =4352a(5) =33792a(6) =266240a(7) =2113536a(8) =16842752a(9) =134479872a(10) =1074790400a(11) =8594128896a(12) =68736253952a(13) =549822922752