a(n) = 4*a(n-1) - 4*a(n-2) + 3*a(n-3).

Terms

a(0) =1a(1) =2a(2) =4a(3) =11a(4) =34a(5) =104a(6) =313a(7) =938a(8) =2812a(9) =8435a(10) =25306a(11) =75920a(12) =227761a(13) =683282a(14) =2049844a(15) =6149531a(16) =18448594a(17) =55345784a(18) =166037353a(19) =498112058a(20) =1494336172a(21) =4483008515a(22) =13449025546a(23) =40347076640a(24) =121041229921a(25) =363123689762