a(n) = 2^n + 6^n.

Terms

a(0) =2a(1) =8a(2) =40a(3) =224a(4) =1312a(5) =7808a(6) =46720a(7) =280064a(8) =1679872a(9) =10078208a(10) =60467200a(11) =362799104a(12) =2176786432a(13) =13060702208a(14) =78364180480a(15) =470185017344