a(n) = C(n,2) + C(n,5) + ... + C(n, 3*floor(n/3)+2).

Terms

a(0) =0a(1) =0a(2) =1a(3) =3a(4) =6a(5) =11a(6) =21a(7) =42a(8) =85a(9) =171a(10) =342a(11) =683a(12) =1365a(13) =2730a(14) =5461a(15) =10923a(16) =21846a(17) =43691a(18) =87381a(19) =174762a(20) =349525a(21) =699051a(22) =1398102a(23) =2796203a(24) =5592405a(25) =11184810a(26) =22369621a(27) =44739243a(28) =89478486a(29) =178956971