Group Homomorphism

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## Intuition

## Types

## Image and kernel

## Examples

## The category of groups

## Homomorphisms of abelian groups

## See also

## References

## External links

This article uses material from the Wikipedia page available here. It is released under the Creative Commons Attribution-Share-Alike License 3.0.

Group Homomorphism

In mathematics, given two groups, (*G*, *) and (*H*, ·), a **group homomorphism** from (*G*, *) to (*H*, ·) is a function *h* : *G* -> *H* such that for all *u* and *v* in *G* it holds that

where the group operation on the left side of the equation is that of *G* and on the right side that of *H*.

From this property, one can deduce that *h* maps the identity element *e _{G}* of

and it also maps inverses to inverses in the sense that

Hence one can say that *h* "is compatible with the group structure".

Older notations for the homomorphism *h*(*x*) may be *x*^{h} or *x*_{h},^{[]} though this may be confused as an index or a general subscript. In automata theory, sometimes homomorphisms are written to the right of their arguments without parentheses, so that *h*(*x*) becomes simply *x h*.^{[]}

In areas of mathematics where one considers groups endowed with additional structure, a *homomorphism* sometimes means a map which respects not only the group structure (as above) but also the extra structure. For example, a homomorphism of topological groups is often required to be continuous.

The purpose of defining a group homomorphism is to create functions that preserve the algebraic structure. An equivalent definition of group homomorphism is: The function *h* : *G* -> *H* is a group homomorphism if whenever

*a* * *b* = *c* we have *h*(*a*) ? *h*(*b*) = *h*(*c*).

In other words, the group *H* in some sense has a similar algebraic structure as *G* and the homomorphism *h* preserves that.

- Monomorphism
- A group homomorphism that is injective (or, one-to-one); i.e., preserves distinctness.
- Epimorphism
- A group homomorphism that is surjective (or, onto); i.e., reaches every point in the codomain.
- Isomorphism
- A group homomorphism that is bijective; i.e., injective and surjective. Its inverse is also a group homomorphism. In this case, the groups
*G*and*H*are called*isomorphic*; they differ only in the notation of their elements and are identical for all practical purposes. - Endomorphism
- A homomorphism,
*h*:*G*->*G*; the domain and codomain are the same. Also called an endomorphism of*G*. - Automorphism
- An endomorphism that is bijective, and hence an isomorphism. The set of all automorphisms of a group
*G*, with functional composition as operation, forms itself a group, the*automorphism group*of*G*. It is denoted by Aut(*G*). As an example, the automorphism group of (**Z**, +) contains only two elements, the identity transformation and multiplication with -1; it is isomorphic to**Z**/2**Z**.

We define the *kernel of h* to be the set of elements in *G* which are mapped to the identity in *H*

and the *image of h* to be

The kernel and image of a homomorphism can be interpreted as measuring how close it is to being an isomorphism. The first isomorphism theorem states that the image of a group homomorphism, *h*(*G*) is isomorphic to the quotient group *G*/ker *h*.

The kernel of h is a normal subgroup of *G* and the image of h is a subgroup of *H*:

If and only if }, the homomorphism, *h*, is a *group monomorphism*; i.e., *h* is injective (one-to-one). Injection directly gives that there is a unique element in the kernel, and a unique element in the kernel gives injection:

- Consider the cyclic group
**Z**/3**Z**= {0, 1, 2} and the group of integers**Z**with addition. The map*h*:**Z**->**Z**/3**Z**with*h*(*u*) =*u*mod 3 is a group homomorphism. It is surjective and its kernel consists of all integers which are divisible by 3.

- Consider the group
For any complex number

*u*the function*f*:_{u}*G*->**C**defined by:^{*} - Consider multiplicative group of positive real numbers (
**R**^{+}, ?) for any complex number*u*the function*f*:_{u}**R**^{+}->**C**defined by:

- The exponential map yields a group homomorphism from the group of real numbers
**R**with addition to the group of non-zero real numbers**R*** with multiplication. The kernel is {0} and the image consists of the positive real numbers. - The exponential map also yields a group homomorphism from the group of complex numbers
**C**with addition to the group of non-zero complex numbers**C*** with multiplication. This map is surjective and has the kernel {2?*ki*:*k*?**Z**}, as can be seen from Euler's formula. Fields like**R**and**C**that have homomorphisms from their additive group to their multiplicative group are thus called exponential fields.

If and are group homomorphisms, then so is . This shows that the class of all groups, together with group homomorphisms as morphisms, forms a category.

If *G* and *H* are abelian (i.e., commutative) groups, then the set of all group homomorphisms from *G* to *H* is itself an abelian group: the sum of two homomorphisms is defined by

- (
*h*+*k*)(*u*) =*h*(*u*) +*k*(*u*) for all*u*in*G*.

The commutativity of *H* is needed to prove that is again a group homomorphism.

The addition of homomorphisms is compatible with the composition of homomorphisms in the following sense: if *f* is in , *h*, *k* are elements of , and *g* is in , then

- and .

Since the composition is associative, this shows that the set End(*G*) of all endomorphisms of an abelian group forms a ring, the *endomorphism ring* of *G*. For example, the endomorphism ring of the abelian group consisting of the direct sum of *m* copies of **Z**/*n***Z** is isomorphic to the ring of *m*-by-*m* matrices with entries in **Z**/*n***Z**. The above compatibility also shows that the category of all abelian groups with group homomorphisms forms a preadditive category; the existence of direct sums and well-behaved kernels makes this category the prototypical example of an abelian category.

- Dummit, D. S.; Foote, R. (2004).
*Abstract Algebra*(3rd ed.). Wiley. pp. 71-72. ISBN 978-0-471-43334-7. - Lang, Serge (2002),
*Algebra*, Graduate Texts in Mathematics,**211**(Revised third ed.), New York: Springer-Verlag, ISBN 978-0-387-95385-4, MR 1878556, Zbl 0984.00001

- Rowland, Todd & Weisstein, Eric W. "Group Homomorphism".
*MathWorld*.

This article uses material from the Wikipedia page available here. It is released under the Creative Commons Attribution-Share-Alike License 3.0.

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