[ n(n-1)(n-2)(n-3)/7 ].

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =3a(5) =17a(6) =51a(7) =120a(8) =240a(9) =432a(10) =720a(11) =1131a(12) =1697a(13) =2451a(14) =3432a(15) =4680a(16) =6240a(17) =8160a(18) =10491a(19) =13289a(20) =16611a(21) =20520a(22) =25080a(23) =30360a(24) =36432a(25) =43371a(26) =51257a(27) =60171a(28) =70200a(29) =81432