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

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =5a(5) =17a(6) =53a(7) =157a(8) =469a(9) =1405a(10) =4217a(11) =12653a(12) =37961a(13) =113881a(14) =341641a(15) =1024921a(16) =3074765a(17) =9224297a(18) =27672893a(19) =83018677a(20) =249056029a(21) =747168085a(22) =2241504257a(23) =6724512773a(24) =20173538321a(25) =60520614961a(26) =181561844881a(27) =544685534641