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

Terms

a(0) =1a(1) =2a(2) =20a(3) =8a(4) =464a(5) =-736a(6) =12608a(7) =-42880a(8) =388352a(9) =-1805824a(10) =12932096a(11) =-69203968a(12) =448778240a(13) =-2558451712a(14) =15887581184a(15) =-93178003456a(16) =567657955328