a(n) = 2a(n-1) + a(n-2) - a(n-3); a(0)=4, a(1)=9, a(2)=20.

Terms

a(0) =4a(1) =9a(2) =20a(3) =45a(4) =101a(5) =227a(6) =510a(7) =1146a(8) =2575a(9) =5786a(10) =13001a(11) =29213a(12) =65641a(13) =147494a(14) =331416a(15) =744685a(16) =1673292a(17) =3759853a(18) =8448313a(19) =18983187a(20) =42654834a(21) =95844542a(22) =215360731a(23) =483911170a(24) =1087338529a(25) =2443227497a(26) =5489882353a(27) =12335653674a(28) =27717962204