a(n) = floor(n*(n-1)*(n-2)*(n-3)/30).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =4a(6) =12a(7) =28a(8) =56a(9) =100a(10) =168a(11) =264a(12) =396a(13) =572a(14) =800a(15) =1092a(16) =1456a(17) =1904a(18) =2448a(19) =3100a(20) =3876a(21) =4788a(22) =5852a(23) =7084a(24) =8500a(25) =10120a(26) =11960a(27) =14040a(28) =16380a(29) =19000