a(n) = 2*a(n-1) + 4*a(n-2); with a(0)=1, a(1)=4.

Terms

a(0) =1a(1) =4a(2) =12a(3) =40a(4) =128a(5) =416a(6) =1344a(7) =4352a(8) =14080a(9) =45568a(10) =147456a(11) =477184a(12) =1544192a(13) =4997120a(14) =16171008a(15) =52330496a(16) =169345024a(17) =548012032a(18) =1773404160a(19) =5738856448a(20) =18571329536a(21) =60098084864a(22) =194481487872a(23) =629355315200