a(n) = Sum_{k=0..floor(n/6)} binomial(n,6*k).

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =1a(5) =1a(6) =2a(7) =8a(8) =29a(9) =85a(10) =211a(11) =463a(12) =926a(13) =1730a(14) =3095a(15) =5461a(16) =9829a(17) =18565a(18) =37130a(19) =77540a(20) =164921a(21) =349525a(22) =728575a(23) =1486675a(24) =2973350a(25) =5858126a(26) =11450531a(27) =22369621a(28) =43942081a(29) =87087001