a(n) = floor(binomial(n,3)/3).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =3a(6) =6a(7) =11a(8) =18a(9) =28a(10) =40a(11) =55a(12) =73a(13) =95a(14) =121a(15) =151a(16) =186a(17) =226a(18) =272a(19) =323a(20) =380a(21) =443a(22) =513a(23) =590a(24) =674a(25) =766a(26) =866a(27) =975a(28) =1092a(29) =1218