a(n) = floor(n*(n-1)*(n-2)*(n-3)/31).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =3a(6) =11a(7) =27a(8) =54a(9) =97a(10) =162a(11) =255a(12) =383a(13) =553a(14) =774a(15) =1056a(16) =1409a(17) =1842a(18) =2369a(19) =3000a(20) =3750a(21) =4633a(22) =5663a(23) =6855a(24) =8226a(25) =9793a(26) =11574a(27) =13587a(28) =15851a(29) =18387