a(n) = 3*a(n-1) - a(n-2), a(0)=1, a(1)=6.

Terms

a(0) =1a(1) =6a(2) =17a(3) =45a(4) =118a(5) =309a(6) =809a(7) =2118a(8) =5545a(9) =14517a(10) =38006a(11) =99501a(12) =260497a(13) =681990a(14) =1785473a(15) =4674429a(16) =12237814a(17) =32039013a(18) =83879225a(19) =219598662a(20) =574916761a(21) =1505151621a(22) =3940538102a(23) =10316462685a(24) =27008849953a(25) =70710087174a(26) =185121411569a(27) =484654147533