a(n) = floor(n*(n-1)*(n-2)/21).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =2a(6) =5a(7) =10a(8) =16a(9) =24a(10) =34a(11) =47a(12) =62a(13) =81a(14) =104a(15) =130a(16) =160a(17) =194a(18) =233a(19) =276a(20) =325a(21) =380a(22) =440a(23) =506a(24) =578a(25) =657a(26) =742a(27) =835a(28) =936a(29) =1044