a(n) = a(n-1) + 4*a(n-2) - 2*a(n-3), where a(0) = 2, a(1) = 4, a(2) = 8.

Terms

a(0) =2a(1) =4a(2) =8a(3) =20a(4) =44a(5) =108a(6) =244a(7) =588a(8) =1348a(9) =3212a(10) =7428a(11) =17580a(12) =40868a(13) =96332a(14) =224644a(15) =528236a(16) =1234148a(17) =2897804a(18) =6777924a(19) =15900844a(20) =37216932a(21) =87264460a(22) =204330500a(23) =478954476a(24) =1121747556a(25) =2628904460a(26) =6157985732a(27) =14430108460a(28) =33804242468a(29) =79208704844