a(n) = 2*a(n-1) - a(n-3) for n >= 3, where a(0) = 2, a(1) = 4, a(2) = 6, a(3) = 8.

Terms

a(0) =2a(1) =4a(2) =6a(3) =8a(4) =12a(5) =18a(6) =28a(7) =44a(8) =70a(9) =112a(10) =180a(11) =290a(12) =468a(13) =756a(14) =1222a(15) =1976a(16) =3196a(17) =5170a(18) =8364a(19) =13532a(20) =21894a(21) =35424a(22) =57316a(23) =92738a(24) =150052a(25) =242788a(26) =392838a(27) =635624a(28) =1028460a(29) =1664082