a(n) = floor(n(n-1)(n-2)(n-3)/19).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =6a(6) =18a(7) =44a(8) =88a(9) =159a(10) =265a(11) =416a(12) =625a(13) =903a(14) =1264a(15) =1724a(16) =2298a(17) =3006a(18) =3865a(19) =4896a(20) =6120a(21) =7560a(22) =9240a(23) =11185a(24) =13422a(25) =15978a(26) =18884a(27) =22168a(28) =25863a(29) =30001