a(n) = Sum_{k=0,1,2,...,n-4,n-2,n-1} a(k); a(n-3) is not a summand, with a(0)=0, a(1)=1, a(2)=3.

Terms

a(0) =0a(1) =1a(2) =3a(3) =4a(4) =7a(5) =12a(6) =23a(7) =43a(8) =81a(9) =151a(10) =282a(11) =526a(12) =982a(13) =1833a(14) =3422a(15) =6388a(16) =11925a(17) =22261a(18) =41556a(19) =77575a(20) =144814a(21) =270333a(22) =504647a(23) =942055a(24) =1758591a(25) =3282868a(26) =6128328a(27) =11440120a(28) =21355963a(29) =39866466