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.

(2)

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