a(n) = floor( n*(n-1)*(n-2)/8 ).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =3a(5) =7a(6) =15a(7) =26a(8) =42a(9) =63a(10) =90a(11) =123a(12) =165a(13) =214a(14) =273a(15) =341a(16) =420a(17) =510a(18) =612a(19) =726a(20) =855a(21) =997a(22) =1155a(23) =1328a(24) =1518a(25) =1725a(26) =1950a(27) =2193a(28) =2457a(29) =2740