a(n) = 3*a(n-1) - a(n-2) for n >= 2, with a(0) = a(1) = 1.

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =13a(5) =34a(6) =89a(7) =233a(8) =610a(9) =1597a(10) =4181a(11) =10946a(12) =28657a(13) =75025a(14) =196418a(15) =514229a(16) =1346269a(17) =3524578a(18) =9227465a(19) =24157817a(20) =63245986a(21) =165580141a(22) =433494437a(23) =1134903170a(24) =2971215073a(25) =7778742049a(26) =20365011074a(27) =53316291173a(28) =139583862445a(29) =365435296162