a(n) = 3*a(n-1) - a(n-3) + 3*a(n-4), with initial values 1,3,9,11.

Terms

a(0) =1a(1) =3a(2) =9a(3) =11a(4) =33a(5) =99a(6) =313a(7) =939a(8) =2817a(9) =8435a(10) =25305a(11) =75915a(12) =227761a(13) =683283a(14) =2049849a(15) =6149531a(16) =18448593a(17) =55345779a(18) =166037353a(19) =498112059a(20) =1494336177a(21) =4483008515a(22) =13449025545a(23) =40347076635a(24) =121041229921a(25) =363123689763