A243210 Number of inequivalent (mod D_3) ways to place 5 points on a triangular grid of side n so that no three of them are vertices of an equilateral triangle with sides parallel to the grid.
0, 11, 266, 2344, 12907, 53307, 180876, 530654, 1391647, 3335627, 7426885, 15544434, 30867669, 58574800, 106838511, 188190111, 321383808, 533857914
Offset: 3
Formula
a(n) = (n^10 + 5*n^9 - 10*n^8 - 195*n^7)/23040 + O(n^6)