a(n) = 2*a(n-1) + 2*a(n-3) - 3*a(n-4), where a(0) = 2, a(1) = 4, a(2) = 8, a(3) = 16.

Terms

a(0) =2a(1) =4a(2) =8a(3) =16a(4) =34a(5) =72a(6) =152a(7) =324a(8) =690a(9) =1468a(10) =3128a(11) =6664a(12) =14194a(13) =30240a(14) =64424a(15) =137244a(16) =292386a(17) =622900a(18) =1327016a(19) =2827072a(20) =6022786a(21) =12830904a(22) =27334904a(23) =58234164a(24) =124061778a(25) =264300652a(26) =563064920a(27) =1199550904a(28) =2555517778a(29) =5444263440