A subset of the elements of a group (written additively) with the property that if a table of all of the differences x-y (with x and y in the distinguished subset) is made, every non-identity element of the group occurs in this table the same number of times. The quadratic residues in GF(11) form a difference set. Suppose that there is a difference set of size k in a group G, then twice the binomial coefficient (k) must be a multiple of |G|-1.

--back to combinatorics--

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