a(1)=4, a(2)=2, a(n) = 4*a(n-1) + 2*a(n-2).

Terms

a(0) =4a(1) =2a(2) =16a(3) =68a(4) =304a(5) =1352a(6) =6016a(7) =26768a(8) =119104a(9) =529952a(10) =2358016a(11) =10491968a(12) =46683904a(13) =207719552a(14) =924246016a(15) =4112423168a(16) =18298184704a(17) =81417585152a(18) =362266710016