Difference set
- For the set of elements in one set but not another, see relative complement. For the set of differences of pairs of elements, see Minkowski difference.
In combinatorics, a difference set is a subset
of size
of a group
of order
such that every nonidentity element of
can be expressed as a product
of elements of
in exactly
ways. A difference set
is said to be cyclic, abelian, non-abelian, etc., if the group
has the corresponding property. A difference set with
is sometimes called planar or simple. If
is an abelian group written in additive notation, the defining condition is that every nonzero element of
can be written as a difference of elements of
in exactly
ways. The term "difference set" arises in this way.