a(n) = Sum_{k=0..floor(n/7)} (-1)^k*binomial(n,7*k).

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =1a(5) =1a(6) =1a(7) =0a(8) =-7a(9) =-35a(10) =-119a(11) =-329a(12) =-791a(13) =-1715a(14) =-3430a(15) =-6419a(16) =-11319a(17) =-18767a(18) =-28763a(19) =-38759a(20) =-38759a(21) =0a(22) =149205a(23) =571781a(24) =1613129a(25) =3964051a(26) =8934121a(27) =18874261a(28) =37748522a(29) =71705865