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

Terms

a(0) =0a(1) =1a(2) =1a(3) =1a(4) =3a(5) =4a(6) =7a(7) =13a(8) =21a(9) =37a(10) =64a(11) =109a(12) =189a(13) =325a(14) =559a(15) =964a(16) =1659a(17) =2857a(18) =4921a(19) =8473a(20) =14592a(21) =25129a(22) =43273a(23) =74521a(24) =128331a(25) =220996a(26) =380575a(27) =655381a(28) =1128621a(29) =1943581