# 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*.^{[1]} 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.

## Basic facts

- A simple counting argument shows that there are exactly pairs of elements from that will yield nonidentity elements, so every difference set must satisfy the equation .
- If is a difference set, and , then is also a difference set, and is called a
**translate**of ( in additive notation). - The complement of a -difference set is a -difference set.
^{[2]} - The set of all translates of a difference set forms a symmetric block design, called the
*development*of and denoted by . In such a design there are*elements*(usually called points) and*blocks*(subsets). Each block of the design consists of points, each point is contained in blocks. Any two blocks have exactly elements in common and any two points are simultaneously contained in exactly blocks. The group acts as an automorphism group of the design. It is sharply transitive on both points and blocks.^{[3]}- In particular, if , then the difference set gives rise to a projective plane. An example of a (7,3,1) difference set in the group is the subset . The translates of this difference set form the Fano plane.

- Since every difference set gives a symmetric design, the parameter set must satisfy the Bruck–Ryser–Chowla theorem.
^{[4]} - Not every symmetric design gives a difference set.
^{[5]}

## Equivalent and isomorphic difference sets

Two difference sets in group and in group are **equivalent** if there is a group isomorphism between and such that for some . The two difference sets are **isomorphic** if the designs and are isomorphic as block designs.

Equivalent difference sets are isomorphic, but there exist examples of isomorphic difference sets which are not equivalent. In the cyclic difference set case, all known isomorphic difference sets are equivalent.^{[6]}

## Multipliers

A **multiplier** of a difference set in group is a group automorphism of such that for some . If is abelian and is the automorphism that maps , then is called a *numerical* or *Hall* **multiplier**.^{[7]}

It has been conjectured that if *p* is a prime dividing and not dividing *v*, then the group automorphism defined by fixes some translate of *D* (this is equivalent to being a multiplier). It is known to be true for when is an abelian group, and this is known as the First Multiplier Theorem. A more general known result, the Second Multiplier Theorem, says that if is a -difference set in an abelian group of exponent (the least common multiple of the orders of every element), let be an integer coprime to . If there exists a divisor of such that for every prime *p* dividing *m*, there exists an integer *i* with , then *t* is a numerical divisor.^{[8]}

For example, 2 is a multiplier of the (7,3,1)-difference set mentioned above.

It has been mentioned that a numerical multiplier of a difference set in an abelian group fixes a translate of , but it can also be shown that there is a translate of which is fixed by all numerical multipliers of .^{[9]}

## Parameters

The known difference sets or their complements have one of the following parameter sets:^{[10]}

- -difference set for some prime power and some positive integer . These are known as the
*classical parameters*and there are many constructions of difference sets having these parameters. - -difference set for some positive integer . Difference sets with
*v*= 4n - 1 are called*Paley-type difference sets*. - -difference set for some positive integer . A difference set with these parameters is a
*Hadamard difference set*. - -difference set for some prime power and some positive integer . Known as the
*McFarland parameters*. - -difference set for some positive integer . Known as the
*Spence parameters*. - -difference set for some prime power and some positive integer . Difference sets with these parameters are called
*Davis-Jedwab-Chen difference sets*.

## Known difference sets

In many constructions of difference sets the groups that are used are related to the additive and multiplicative groups of finite fields. The notation used to denote these fields differs according to discipline. In this section, is the Galois field of order , where is a prime or prime power. The group under addition is denoted by , while is the multiplicative group of non-zero elements.

- Let . Then the set is a -difference set, where is the trace function .

## History

The systematic use of cyclic difference sets and methods for the construction of symmetric block designs dates back to R. C. Bose and a seminal paper of his in 1939.^{[12]} However, various examples appeared earlier than this, such as the "Paley Difference Sets" which date back to 1933.^{[13]} The generalization of the cyclic difference set concept to more general groups is due to R.H. Bruck^{[14]} in 1955.^{[15]} Multipliers were introduced by Marshall Hall Jr.^{[16]} in 1947.^{[17]}

## Application

It is found by Xia, Zhou and Giannakis that, difference sets can be used to construct a complex vector codebook that achieves the difficult Welch bound on maximum cross correlation amplitude. The so-constructed codebook also forms the so-called Grassmannian manifold.

## Generalisations

A **difference family** is a set of subsets of a group such that the order of is , the size of is for all , and every nonidentity element of can be expressed as a product of elements of for some (i.e. both come from the same ) in exactly ways.

A difference set is a difference family with . The parameter equation above generalises to .^{[18]}
The development of a difference family is a 2-design.
Every 2-design with a regular automorphism group is for some difference family .

## See also

## Notes

- ↑ Template:Harvnb
- ↑ Template:Harvnb
- ↑ Template:Harvnb. The theorem only states point transitivity, but block transitivity follows from this by the second corollary on p. 330.
- ↑ Template:Harvnb
- ↑ Template:Harvnb
- ↑ Template:Harvnb
- ↑ Template:Harvnb
- ↑ Template:Harvnb
- ↑ Template:Harvnb
- ↑ Template:Harvnb
- ↑ Template:Harvnb
- ↑ {{#invoke:citation/CS1|citation |CitationClass=citation }}
- ↑ Template:Harvnb
- ↑ {{#invoke:citation/CS1|citation |CitationClass=citation }}
- ↑ Template:Harvnb
- ↑ {{#invoke:citation/CS1|citation |CitationClass=citation }}
- ↑ Template:Harvnb
- ↑ Template:Harvnb

## References

- {{#invoke:citation/CS1|citation

|CitationClass=citation }}

- {{#invoke:citation/CS1|citation

|CitationClass=citation }}

- {{#invoke:citation/CS1|citation

|CitationClass=book }}

- {{#invoke:citation/CS1|citation

|CitationClass=citation }}

- {{#invoke:citation/CS1|citation

|CitationClass=book }}

- {{#invoke:citation/CS1|citation

|CitationClass=book }}

- {{#invoke:Citation/CS1|citation

|CitationClass=journal }}.

- {{#invoke:Citation/CS1|citation

|CitationClass=journal }}