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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