Let be a Group of Order and be a set of elements of . If the set of differences contains every Nonzero element of exactly times, then is a -difference set in of Order . If , the difference set is called planar. The quadratic residues in the Galois Field form a difference set. If there is a difference set of size in a group , then must be a multiple of , where is a Binomial Coefficient.

**References**

Gordon, D. M. ``The Prime Power Conjecture is True for .'' *Electronic J. Combinatorics* **1**, R6 1-7, 1994.
http://www.combinatorics.org/Volume_1/volume1.html#R6.

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1999-05-24