Number of non-equivalent ways to choose 4 points in an equilateral triangle grid of side n.

Terms

a(0) =0a(1) =0a(2) =4a(3) =41a(4) =244a(5) =1029a(6) =3485a(7) =9926a(8) =25030a(9) =57126a(10) =120570a(11) =238330a(12) =446344a(13) =797825a(14) =1370684a(15) =2274259a(16) =3660612a(17) =5734776a(18) =8771181a(19) =13127940a(20) =19270240a(21) =27789713a(22) =39435814a(23) =55142010a(24) =76066910a(25) =103627784a(26) =139554142a(27) =185929971a(28) =245260890a(29) =320527585