a(n) = floor( n*(n-1)*(n-2)/27 ).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =2a(6) =4a(7) =7a(8) =12a(9) =18a(10) =26a(11) =36a(12) =48a(13) =63a(14) =80a(15) =101a(16) =124a(17) =151a(18) =181a(19) =215a(20) =253a(21) =295a(22) =342a(23) =393a(24) =449a(25) =511a(26) =577a(27) =650a(28) =728a(29) =812