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### Homomorphisms

Homomorphisms are an important part of group theory, and Magma supports homomorphisms between groups. Many useful homomorphisms are returned by constructors and intrinsic functions. Examples of these are the quo constructor, the sub constructor and intrinsic functions such as OrbitAction and FPGroup, which are described in more detail elsewhere in this chapter. In this section we describe how the user may create their own homomorphisms with domain a matrix group.

##### hom<G | L> : GrpMat, List -> Map
Given the matrix group G, construct the homomorphism f : G to H given by the generator images in L. H must be a group. The clause L may be any one of the following types:
• A list of elements of H, giving images of the generators of G;
• A list of pairs, where the first in the pair is an element of G and the second its image in H;
• A sequence of elements of H, as in (a);
• A set or sequence of pairs, as in (b);

Each image element specified by the list must belong to the same group H. In the cases where pairs are given the given elements of G must generate G.
##### Domain(f) : Map -> Grp
The domain of the homomorphism f.
##### Codomain(f) : Map -> Grp
The codomain of the homomorphism f.
##### Image(f) : Map -> Grp
The image or range of the homomorphism f. This will be a subgroup of the codomain of f. The algorithm computes the image and kernel simultaneously (see [CLG91]).
##### Kernel(f) : Map -> Grp
The kernel of the homomorphism f. This will be a normal subgroup of the domain of f. The algorithm computes the image and kernel simultaneously (see [CLG91]).
##### IsHomomorphism(G, H, Q) : GrpMat, GrpMat, SeqEnum[GrpMatElt] -> Bool, Map
Return the value true if the sequence Q defines a homomorphism from the group G to the group H. The sequence Q must have length Ngens(G) and must contain elements of H. The i-th element of Q is interpreted as the image of the i-th generator of G and the function decides if these images extend to a homomorphism. If so, the homomorphism is also returned.

### Example GrpMat_Homomorphism (H21E7)

We construct the usual degree 2 matrix representation of the dihedral group of order 20, and a homomorphism from it to the symmetric group of degree 5.

```> K<z> := CyclotomicField(20);
> zz := RootOfUnity(10, K);
> i := RootOfUnity(4, K);
> cos := (zz+ComplexConjugate(zz))/2;
> sin := (zz-ComplexConjugate(zz))/(2*i);
> gl := GeneralLinearGroup(2, K);
> M := sub< gl | [cos, sin, -sin, cos], [-1,0,0,1]>;
> #M;
20
> S := SymmetricGroup(5);
> f := hom<M->S |[S|(1,2,3,4,5), (1,5)(2,4)]>;
> Codomain(f);
Symmetric group S acting on a set of cardinality 5
Order = 120 = 2^3 * 3 * 5
> Image(f);
Permutation group acting on a set of cardinality 5
Order = 10 = 2 * 5
(1, 2, 3, 4, 5)
(1, 5)(2, 4)
> Kernel(f);
MatrixGroup(2, K) of order 2
Generators:
[-1  0]
[ 0 -1]
```

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