a(n) = C(n,0) + C(n,3) + ... + C(n,3[n/3]).

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =5a(5) =11a(6) =22a(7) =43a(8) =85a(9) =170a(10) =341a(11) =683a(12) =1366a(13) =2731a(14) =5461a(15) =10922a(16) =21845a(17) =43691a(18) =87382a(19) =174763a(20) =349525a(21) =699050a(22) =1398101a(23) =2796203a(24) =5592406a(25) =11184811a(26) =22369621a(27) =44739242a(28) =89478485a(29) =178956971