a(n) = floor(n(n-1)(n-2)(n-3)/18).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =6a(6) =20a(7) =46a(8) =93a(9) =168a(10) =280a(11) =440a(12) =660a(13) =953a(14) =1334a(15) =1820a(16) =2426a(17) =3173a(18) =4080a(19) =5168a(20) =6460a(21) =7980a(22) =9753a(23) =11806a(24) =14168a(25) =16866a(26) =19933a(27) =23400a(28) =27300a(29) =31668