a(n) = Sum_{k=0..floor(4*n/7)} binomial(k+2,4*n-7*k).

Terms

a(0) =1a(1) =0a(2) =3a(3) =0a(4) =6a(5) =0a(6) =10a(7) =1a(8) =15a(9) =7a(10) =21a(11) =28a(12) =28a(13) =84a(14) =37a(15) =210a(16) =56a(17) =462a(18) =121a(19) =924a(20) =352a(21) =1717a(22) =1079a(23) =3018a(24) =3094a(25) =5125a(26) =8113a(27) =8688a(28) =19569a(29) =15436