a(n) = floor(n*(n-1)*(n-2)/7).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =3a(5) =8a(6) =17a(7) =30a(8) =48a(9) =72a(10) =102a(11) =141a(12) =188a(13) =245a(14) =312a(15) =390a(16) =480a(17) =582a(18) =699a(19) =830a(20) =977a(21) =1140a(22) =1320a(23) =1518a(24) =1734a(25) =1971a(26) =2228a(27) =2507a(28) =2808a(29) =3132