## You are here

Homemixed group

## Primary tabs

# mixed group

A *mixed group* is a partial groupoid $G$ such that $G$ contains a non-empty subset $K$, called the *kernel* of $G$, with the following conditions:

1. if $a,b\in G$, then $ab$ is defined iff $a\in K$,

2. if $a,b\in K$ and $c\in G$, then $(ab)c=a(bc)$,

3. if $a\in K$, then $K\subseteq aK\cap Ka$,

4. if $a\in K$ and $b\in G$ such that $ab=b$, then $ac=c$ for all $c\in G$.

Mixed groups are generalizations of groups, as the following proposition illustrates:

###### Proposition 1.

If $K=G$, then $G$ is a group.

###### Proof.

Now, by condition 3, given $a\in G$, there is $b\in G$ such that $ba=a$, so that $bc=c$ for all $c\in G$ by condition 4. In other words, $b$ is a left identity of $G$. Again, by condition 3, for every $a\in G$, there is a $d\in G$ such that $b=da$. So $ad=a(bd)=a(da)d=(ad)^{2}$, so, by condition 4, $adx=x$ for all $x\in G$. In particular, set $x=a$, we get $a=(ad)a=a(da)=ab$. Hence, $b$ is a two-sided identity, and $G$ is a monoid.

For a non-trivial example of a mixed group, let $G$ be a group and $H$ a subgroup of $G$. Define a new multiplication $\cdot$ on $G$ as follows: $a\cdot b$ is defined iff $a\in H$, and if $a\cdot b$ is defined, it is defined as $ab$, the group multiplication of $a$ and $b$. Then $(G,\cdot)$ is a mixed group. Clearly, associativity of $\cdot$ is automatically satisfied. Next, pick any $a\in H$, then, for any $b\in H$, $a^{{-1}}\cdot b$ and $b\cdot a^{{-1}}$ are both elements of $H$, so that $b\in a\cdot H\cap H\cdot a$, and condition 3 is also satisfied. Finally, if $a\in H$ and $b\in G$ such that $a\cdot b=b$, then $a$ is the multiplicative identity of $G$, clearly $a\cdot c=c$ for all $c\in G$.

# References

- 1 R. H. Bruck, A Survey of Binary Systems, Springer-Verlag, 1966
- 2 R. Baer, Zur Einordnung der Theorie der Mischgruppen in die Gruppentheorie, S.-B. Heidelberg. Akad. Wiss., Math.-naturwiss. KI. 1928, 4, 13 pp
- 3 R. Baer, Über die Zerlegungen einer Mischgruppe nach einer Untermischgruppe, S.-B. Heidelberg. Akad. Wiss., Math.-naturwiss. KI. 1928, 5, 13 pp
- 4 A. Loewy, Über abstrakt definierte Transmutationssysteme oder Mischgruppen, J. reine angew. Math. 157, pp 239-254, 1927

## Mathematics Subject Classification

20N99*no label found*

- Forums
- Planetary Bugs
- HS/Secondary
- University/Tertiary
- Graduate/Advanced
- Industry/Practice
- Research Topics
- LaTeX help
- Math Comptetitions
- Math History
- Math Humor
- PlanetMath Comments
- PlanetMath System Updates and News
- PlanetMath help
- PlanetMath.ORG
- Strategic Communications Development
- The Math Pub
- Testing messages (ignore)

- Other useful stuff
- Corrections