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

Terms

a(0) =2a(1) =4a(2) =8a(3) =18a(4) =40a(5) =92a(6) =210a(7) =484a(8) =1112a(9) =2562a(10) =5896a(11) =13580a(12) =31266a(13) =72004a(14) =165800a(15) =381810a(16) =879208a(17) =2024636a(18) =4662258a(19) =10736164a(20) =24722936a(21) =56931426a(22) =131100232a(23) =301894508a(24) =695195202a(25) =1600878724a(26) =3686464328a(27) =8489100498a(28) =19548493480a(29) =45015794972