a(n) = floor(n*(n-1)*(n-2)/13).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =4a(6) =9a(7) =16a(8) =25a(9) =38a(10) =55a(11) =76a(12) =101a(13) =132a(14) =168a(15) =210a(16) =258a(17) =313a(18) =376a(19) =447a(20) =526a(21) =613a(22) =710a(23) =817a(24) =934a(25) =1061a(26) =1200a(27) =1350a(28) =1512a(29) =1686