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

Terms

a(0) =1a(1) =2a(2) =5a(3) =10a(4) =21a(5) =44a(6) =91a(7) =190a(8) =395a(9) =822a(10) =1711a(11) =3560a(12) =7409a(13) =15418a(14) =32085a(15) =66770a(16) =138949a(17) =289156a(18) =601739a(19) =1252230a(20) =2605915a(21) =5422958a(22) =11285279a(23) =23484880a(24) =48872481a(25) =101704562a(26) =211649125a(27) =440445850a(28) =916576181a(29) =1907412444