Note that the axes of coordinates i and j have a relative angle of 60 degrees. The distance between the cell centers can be expressed by invoking Pythagoras for a triangle with one angle of 60 degrees.
Moreover, we use that the distance between the centers of two neighboring cells is SQRT(3) times the size of an edge R of the hexagonal cell. This gives
ru = SQRT {( i2 + 2ijcos(pi/3) + j2)} SQRT{3} R.
3 SQRT{3} SR = ---------- R2 2The surface area of a cluster of C cells is
3 SQRT{3} ru2 Sc = ---------- ---- 2 3Combining this gives ru = SQRT(3 C) R
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