a(n) = floor(n*(n-1)*(n-2)/4).

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =6a(5) =15a(6) =30a(7) =52a(8) =84a(9) =126a(10) =180a(11) =247a(12) =330a(13) =429a(14) =546a(15) =682a(16) =840a(17) =1020a(18) =1224a(19) =1453a(20) =1710a(21) =1995a(22) =2310a(23) =2656a(24) =3036a(25) =3450a(26) =3900a(27) =4387a(28) =4914a(29) =5481