a(n) = 2*a(n-1) + 2*a(n-2) + 2*a(n-3) - a(n-4); a(0)=0, a(1)=1, a(2)=3, a(3)=9, a(4)=25.

Terms

a(0) =0a(1) =1a(2) =3a(3) =9a(4) =25a(5) =73a(6) =211a(7) =609a(8) =1761a(9) =5089a(10) =14707a(11) =42505a(12) =122841a(13) =355017a(14) =1026019a(15) =2965249a(16) =8569729a(17) =24766977a(18) =71577891a(19) =206863945a(20) =597847897a(21) =1727812489a(22) =4993470771a(23) =14431398369a(24) =41707515361a(25) =120536956513a(26) =348358269715