a(n) = floor( n*(n-1)*(n-2)/28 ).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =2a(6) =4a(7) =7a(8) =12a(9) =18a(10) =25a(11) =35a(12) =47a(13) =61a(14) =78a(15) =97a(16) =120a(17) =145a(18) =174a(19) =207a(20) =244a(21) =285a(22) =330a(23) =379a(24) =433a(25) =492a(26) =557a(27) =626a(28) =702a(29) =783