a(n) = floor(n*(n-1)*(n-2)*(n-3)/9).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =2a(5) =13a(6) =40a(7) =93a(8) =186a(9) =336a(10) =560a(11) =880a(12) =1320a(13) =1906a(14) =2669a(15) =3640a(16) =4853a(17) =6346a(18) =8160a(19) =10336a(20) =12920a(21) =15960a(22) =19506a(23) =23613a(24) =28336a(25) =33733a(26) =39866a(27) =46800a(28) =54600a(29) =63336