a(n) = Sum_{k=0..floor(n/6)} (-1)^k*binomial(n,6*k).

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =1a(5) =1a(6) =0a(7) =-6a(8) =-27a(9) =-83a(10) =-209a(11) =-461a(12) =-922a(13) =-1702a(14) =-2911a(15) =-4549a(16) =-6187a(17) =-6187a(18) =0a(19) =23238a(20) =87021a(21) =238337a(22) =564719a(23) =1217483a(24) =2434966a(25) =4542526a(26) =7865521a(27) =12403951a(28) =16942381a(29) =16942381