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

Terms

a(0) =0a(1) =1a(2) =4a(3) =7a(4) =28a(5) =85a(6) =232a(7) =739a(8) =2200a(9) =6505a(10) =19756a(11) =59071a(12) =176884a(13) =531901a(14) =1594192a(15) =4781851a(16) =14351536a(17) =43044817a(18) =129136084a(19) =387434359a(20) =1162245964a(21) =3486773797a(22) =10460420920a(23) =31380955987a(24) =94143182920a(25) =282429839161a(26) =847287991804