a(n) = Sum_{k=0..floor((2*n+1)/7)} binomial(2*k+1,2*n-7*k+1).

Terms

a(0) =1a(1) =0a(2) =0a(3) =1a(4) =3a(5) =0a(6) =0a(7) =5a(8) =10a(9) =1a(10) =1a(11) =21a(12) =35a(13) =7a(14) =9a(15) =84a(16) =126a(17) =37a(18) =56a(19) =330a(20) =462a(21) =178a(22) =297a(23) =1287a(24) =1717a(25) =820a(26) =1443a(27) =5006a(28) =6452a(29) =3683