a(n) = (4*8^n + (-2)^n)/5.

Terms

a(0) =1a(1) =6a(2) =52a(3) =408a(4) =3280a(5) =26208a(6) =209728a(7) =1677696a(8) =13421824a(9) =107374080a(10) =858993664a(11) =6871947264a(12) =54975582208a(13) =439804649472