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

Terms

a(0) =4a(1) =4a(2) =32a(3) =144a(4) =704a(5) =3392a(6) =16384a(7) =79104a(8) =381952a(9) =1844224a(10) =8904704a(11) =42995712a(12) =207601664a(13) =1002389504a(14) =4839964672a(15) =23369416704a(16) =112837525504a(17) =544827768832