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

Terms

a(0) =1a(1) =4a(2) =19a(3) =97a(4) =505a(5) =2629a(6) =13609a(7) =69997a(8) =357985a(9) =1822309a(10) =9241849a(11) =46731997a(12) =235754065a(13) =1187152789a(14) =5969306089a(15) =29980723597a(16) =150440439745a(17) =754349584069