a(n) = Sum_{k=0..floor(2*n/7)} binomial(3*k,2*n-7*k).

Terms

a(0) =1a(1) =0a(2) =0a(3) =0a(4) =3a(5) =1a(6) =0a(7) =1a(8) =15a(9) =15a(10) =1a(11) =9a(12) =84a(13) =126a(14) =37a(15) =67a(16) =495a(17) =924a(18) =510a(19) =521a(20) =3004a(21) =6436a(22) =5158a(23) =4425a(24) =18669a(25) =43780a(26) =45088a(27) =38913a(28) =119341a(29) =294359