# hypergroup

Definition. A hypergroupoid, or multigroupoid, is a non-empty set $G$, together with a multivalued function $\cdot:G\times G\Rightarrow G$ called the multiplication on $G$.

We write $a\cdot b$, or simply $ab$, instead of $\cdot(a,b)$. Furthermore, if $ab=\{c\}$, then we use the abbreviation $ab=c$.

A hypergroupoid is said to be commutative   if $ab=ba$ for all $a,b\in G$. Defining associativity of $\cdot$ on $G$, however, is trickier:

Given a hypergroupoid $G$, the multiplication $\cdot$ induces a binary operation (also written $\cdot$) on $P(G)$, the powerset of $P$, given by

 $A\cdot B:=\bigcup\{a\cdot b\mid a\in A\mbox{ and }b\in B\}.$

As a result, we have an induced groupoid    $P(G)$. Instead of writing $\{a\}B$, $A\{b\}$, and $\{a\}\{b\}$, we write $aB,Ab$, and $ab$ instead. From now on, when we write $(ab)c$, we mean “first, take the product  of $a$ and $b$ via the multivalued binary operation $\cdot$ on $G$, then take the product of the set $ab$ with the element $c$, under the induced binary operation on $P(G)$”. Given a hypergroupoid $G$, there are two types of associativity we may define:

Type 1.

$(ab)c\subseteq a(bc)$, and

Type 2.

$a(bc)\subseteq(ab)c$.

$G$ is said to be associative if it satisfies both types of associativity laws. An associative hypergroupoid is called a hypersemigroup. We are now ready to formally define a hypergroup.

Definition. A hypergroup is a hypersemigroup $G$ such that $aG=Ga=G$ for all $a\in G$.

For example, let $G$ be a group and $H$ a subgroup   of $G$. Let $M$ be the collection  of all left cosets  of $H$ in $G$. For $aH,bH\in M$, set

 $aH\cdot bH:=\{cH\mid c=ahb\mbox{, }h\in H\}.$

Then $M$ is a hypergroup with multiplication $\cdot$.

If the multiplication in a hypergroup $G$ is single-valued, then $G$ is a $2$-group (http://planetmath.org/PolyadicSemigroup), and therefore a group (see proof here (http://planetmath.org/PolyadicSemigroup)).

Let $G$ be a hypergroupoid. Identity elements are defined via the following three sets:

1. 1.

(set of left identities): $I_{L}(G):=\{e\in G\mid a\in ea\mbox{ for all }a\in G\}$,

2. 2.

(set of right identities): $I_{R}(G):=\{e\in G\mid a\in ae\mbox{ for all }a\in G\}$, and

3. 3.

(set of identities): $I(G)=I_{L}(G)\cap I_{R}(G)$.

$e\in L(G)$ is called an absolute identity if $ea=ae=a$ for all $a\in G$. If $e,f\in G$ are both absolute identities, then $e=ef=f$, so $G$ can have at most one absolute identity.

Suppose $e\in I_{L}(G)\cup I_{R}(G)$ and $a\in G$. An element $b\in G$ is said to be a left inverse of $a$ with respect to $e$ if $e\in ba$. Right inverses of $a$ are defined similarly. If $b$ is both a left and a right inverse of $a$ with respect to $e$, then $b$ is called an inverse of $a$ with respect to $e$.

Thus, one may say that a multigroup is a hypergroup $G$ with an identity $e\in G$, and a function ${}^{-1}:G\to G$ such that $a^{-1}:=^{-1}(a)$ is an inverse of $a$ with respect to $e$.

In the example above, $M$ is a multigroup in the sense given in the remark above. The designated identity is $H$ (in fact, this is the only identity in $M$), and for every $aH\in M$, its designated inverse is provided by $a^{-1}H$ (of course, this may not be its only inverse, as any $bH$ such that $ahb=e$ for some $h\in H$ will do).

## References

• 1 R. H. Bruck, A Survey on Binary Systems, Springer-Verlag, New York, 1966.
• 2 M. Dresher, O. Ore, Theory of Multigroups, Amer. J. Math. vol. 60, pp. 705-733, 1938.
• 3 J.E. Eaton, O. Ore, Remarks on Multigroups, Amer. J. Math. vol. 62, pp. 67-71, 1940.
• 4 L. W. Griffiths, On Hypergroups, Multigroups, and Product Systems, Amer. J. Math. vol. 60, pp. 345-354, 1938.
• 5 A. P. Dičman, On Multigroups whose Elements are Subsets of a Group, Moskov. Gos. Ped. Inst. Uč. Zap. vol. 71, pp. 71-79, 1953
 Title hypergroup Canonical name Hypergroup Date of creation 2013-03-22 18:38:22 Last modified on 2013-03-22 18:38:22 Owner CWoo (3771) Last modified by CWoo (3771) Numerical id 9 Author CWoo (3771) Entry type Definition Classification msc 20N20 Synonym multigroupoid Synonym multisemigroup Synonym multigroup Related topic group Defines hypergroupoid Defines hypersemigroup Defines left identity Defines right identity Defines identity Defines absolute identity Defines left inverse Defines right inverse Defines inverse Defines absolute identity