a(n) = floor( n*(n-1)*(n-2)/14 ).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =4a(6) =8a(7) =15a(8) =24a(9) =36a(10) =51a(11) =70a(12) =94a(13) =122a(14) =156a(15) =195a(16) =240a(17) =291a(18) =349a(19) =415a(20) =488a(21) =570a(22) =660a(23) =759a(24) =867a(25) =985a(26) =1114a(27) =1253a(28) =1404a(29) =1566