a(n) = a(n-1) + a(n-2) - a(n-4) starting a(0)=0, a(1)=1, a(2)=a(3)=3.

Terms

a(0) =0a(1) =1a(2) =3a(3) =3a(4) =6a(5) =8a(6) =11a(7) =16a(8) =21a(9) =29a(10) =39a(11) =52a(12) =70a(13) =93a(14) =124a(15) =165a(16) =219a(17) =291a(18) =386a(19) =512a(20) =679a(21) =900a(22) =1193a(23) =1581a(24) =2095a(25) =2776a(26) =3678a(27) =4873a(28) =6456a(29) =8553