a(n) = (4*6^n + (-4)^n)/5.

Terms

a(0) =1a(1) =4a(2) =32a(3) =160a(4) =1088a(5) =6016a(6) =38144a(7) =220672a(8) =1356800a(9) =8009728a(10) =48582656a(11) =289398784a(12) =1744781312a(13) =10435133440a(14) =62745018368a(15) =375933239296