a(n) = (Sum{k=0..n-1} a(k)) - a(n-3), with a(0)=0, a(1)=0, a(2)=1.

Terms

a(0) =0a(1) =0a(2) =1a(3) =1a(4) =2a(5) =3a(6) =6a(7) =11a(8) =21a(9) =39a(10) =73a(11) =136a(12) =254a(13) =474a(14) =885a(15) =1652a(16) =3084a(17) =5757a(18) =10747a(19) =20062a(20) =37451a(21) =69912a(22) =130509a(23) =243629a(24) =454797a(25) =848997a(26) =1584874a(27) =2958580a(28) =5522960a(29) =10310043