an even power of an odd non-Pythagorean prime. This choice of q implies that in the unique finite field F_q of order q, the element -1 has a square root.

Now let V = F_q and let
:.If a pair {a,b} is included in E, it is included under either ordering of its two elements. For, a - b = -(b - a), and -1 is a square, from which it follows that a - b is a square if and only if b - a is a square.