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 eG of G to the identity element eH of H, and it also maps inverses to inverses in the sense that Hence one can say that h "is compatible with the group structure". 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.

Properties

Let e_{H} be the identity element of the (H, ·) group and u \in G, then Now by multiplying for the inverse of h(u) (or applying the cancellation rule) we obtain Similarly, Therefore for the uniqueness of the inverse:.

Types

Image and kernel

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. Assume and show for arbitrary u, g: The image of h is a subgroup of H. The homomorphism, h, is a group monomorphism; i.e., h is injective (one-to-one) if and only if. Injection directly gives that there is a unique element in the kernel, and, conversely, a unique element in the kernel gives injection:

Examples

• The set forms a group under matrix multiplication. For any complex number u the function fu : G → C* defined by is a group homomorphism. • Consider a multiplicative group of positive real numbers (R+, ⋅) for any complex number u. Then the function fu : R+ → C defined by is a group homomorphism.

Category of groups

If h : G → H and k : H → K are group homomorphisms, then so is k ∘ h : G → K. This shows that the class of all groups, together with group homomorphisms as morphisms, forms a category (specifically the category of groups).

Homomorphisms of abelian groups

If G and H are abelian (i.e., commutative) groups, then the set Hom(G, H) of all group homomorphisms from G to H is itself an abelian group: the sum h + k of two homomorphisms is defined by The commutativity of H is needed to prove that h + k is again a group homomorphism. The addition of homomorphisms is compatible with the composition of homomorphisms in the following sense: if f is in Hom(K, G), h, k are elements of Hom(G, H), and g is in Hom(H, L), then 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/nZ is isomorphic to the ring of m-by-m matrices with entries in Z/nZ. 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.