a(n) = floor(n(n-1)(n-2)(n-3)/20).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =6a(6) =18a(7) =42a(8) =84a(9) =151a(10) =252a(11) =396a(12) =594a(13) =858a(14) =1201a(15) =1638a(16) =2184a(17) =2856a(18) =3672a(19) =4651a(20) =5814a(21) =7182a(22) =8778a(23) =10626a(24) =12751a(25) =15180a(26) =17940a(27) =21060a(28) =24570a(29) =28501