[ n(n-1)(n-2)(n-3)/13 ].

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =9a(6) =27a(7) =64a(8) =129a(9) =232a(10) =387a(11) =609a(12) =913a(13) =1320a(14) =1848a(15) =2520a(16) =3360a(17) =4393a(18) =5649a(19) =7155a(20) =8944a(21) =11049a(22) =13504a(23) =16347a(24) =19617a(25) =23353a(26) =27600a(27) =32400a(28) =37800a(29) =43848