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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.
4
0, 11, 266, 2344, 12907, 53307, 180876, 530654, 1391647, 3335627, 7426885, 15544434, 30867669, 58574800, 106838511, 188190111, 321383808, 533857914
OFFSET
3,2
FORMULA
a(n) = (n^10 + 5*n^9 - 10*n^8 - 195*n^7)/23040 + O(n^6)
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Heinrich Ludwig, Jun 10 2014
STATUS
approved