a(n) = floor( n*(n-1)*(n-2)*(n-3)/28 ).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =4a(6) =12a(7) =30a(8) =60a(9) =108a(10) =180a(11) =282a(12) =424a(13) =612a(14) =858a(15) =1170a(16) =1560a(17) =2040a(18) =2622a(19) =3322a(20) =4152a(21) =5130a(22) =6270a(23) =7590a(24) =9108a(25) =10842a(26) =12814a(27) =15042a(28) =17550a(29) =20358