a(n) = floor(n*(n-1)*(n-2)*(n-3)/15).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =8a(6) =24a(7) =56a(8) =112a(9) =201a(10) =336a(11) =528a(12) =792a(13) =1144a(14) =1601a(15) =2184a(16) =2912a(17) =3808a(18) =4896a(19) =6201a(20) =7752a(21) =9576a(22) =11704a(23) =14168a(24) =17001a(25) =20240a(26) =23920a(27) =28080a(28) =32760a(29) =38001