# Definition:Homomorphism (Abstract Algebra)

*This page is about Homomorphism in the context of Abstract Algebra. For other uses, see Homomorphism.*

## Definition

Let $\struct {S, \circ}$ and $\struct {T, *}$ be algebraic structures.

Let $\phi: \struct {S, \circ} \to \struct {T, *}$ be a mapping from $\struct {S, \circ}$ to $\struct {T, *}$.

Let $\circ$ have the morphism property under $\phi$, that is:

- $\forall x, y \in S: \map \phi {x \circ y} = \map \phi x * \map \phi y$

Then $\phi$ is a **homomorphism**.

This can be generalised to algebraic structures with more than one operation:

Let:

- $\struct {S_1, \circ_1, \circ_2, \ldots, \circ_n}$
- $\struct {T, *_1, *_2, \ldots, *_n}$

Let $\phi: \struct {S_1, \circ_1, \circ_2, \ldots, \circ_n} \to \struct {T, *_1, *_2, \ldots, *_n}$ be a mapping from $\struct {S_1, \circ_1, \circ_2, \ldots, \circ_n}$ to $\struct {T, *_1, *_2, \ldots, *_n}$.

Let, $\forall k \in \closedint 1 n$, $\circ_k$ have the morphism property under $\phi$, that is:

- $\forall x, y \in S: \map \phi {x \circ_k y} = \map \phi x *_k \map \phi y$

Then $\phi$ is a **homomorphism**.

### Semigroup Homomorphism

Let $\struct {S, \circ}$ and $\struct {T, *}$ be semigroups.

Let $\phi: S \to T$ be a mapping such that $\circ$ has the morphism property under $\phi$.

That is, $\forall a, b \in S$:

- $\map \phi {a \circ b} = \map \phi a * \map \phi b$

Then $\phi: \struct {S, \circ} \to \struct {T, *}$ is a **semigroup homomorphism**.

### Monoid Homomorphism

Let $\struct {S, \circ}$ and $\struct {T, *}$ be monoids.

Let $\phi: S \to T$ be a mapping such that $\circ$ has the morphism property under $\phi$.

That is, $\forall a, b \in S$:

- $\map \phi {a \circ b} = \map \phi a * \map \phi b$

Suppose further that $\phi$ preserves identities, that is:

- $\map \phi {e_S} = e_T$

Then $\phi: \struct {S, \circ} \to \struct {T, *}$ is a **monoid homomorphism**.

### Group Homomorphism

Let $\struct {G, \circ}$ and $\struct {H, *}$ be groups.

Let $\phi: G \to H$ be a mapping such that $\circ$ has the morphism property under $\phi$.

That is, $\forall a, b \in G$:

- $\map \phi {a \circ b} = \map \phi a * \map \phi b$

Then $\phi: \struct {G, \circ} \to \struct {H, *}$ is a **group homomorphism**.

### Ring Homomorphism

Let $\struct {R, +, \circ}$ and $\struct {S, \oplus, *}$ be rings.

Let $\phi: R \to S$ be a mapping such that both $+$ and $\circ$ have the morphism property under $\phi$.

That is, $\forall a, b \in R$:

\(\text {(1)}: \quad\) | \(\ds \map \phi {a + b}\) | \(=\) | \(\ds \map \phi a \oplus \map \phi b\) | |||||||||||

\(\text {(2)}: \quad\) | \(\ds \map \phi {a \circ b}\) | \(=\) | \(\ds \map \phi a * \map \phi b\) |

Then $\phi: \struct {R, +, \circ} \to \struct {S, \oplus, *}$ is a **ring homomorphism**.

### Field Homomorphism

Let $\struct {F, +, \times}$ and $\struct {K, \oplus, \otimes}$ be fields.

Let $\phi: F \to K$ be a mapping such that both $+$ and $\times$ have the morphism property under $\phi$.

That is, $\forall a, b \in F$:

\(\text {(1)}: \quad\) | \(\ds \map \phi {a + b}\) | \(=\) | \(\ds \map \phi a \oplus \map \phi b\) | |||||||||||

\(\text {(2)}: \quad\) | \(\ds \map \phi {a \times b}\) | \(=\) | \(\ds \map \phi a \otimes \map \phi b\) |

Then $\phi: \struct {F, +, \times} \to \struct {K, \oplus, \otimes}$ is a **field homomorphism**.

### $F$-Homomorphism

Let $R, S$ be rings with unity.

Let $F$ be a subfield of both $R$ and $S$.

Then a ring homomorphism $\varphi: R \to S$ is called an **$F$-homomorphism** if:

- $\forall a \in F: \map \phi a = a$

That is, $\phi \restriction_F = I_F$ where:

- $\phi \restriction_F$ is the restriction of $\phi$ to $F$
- $I_F$ is the identity mapping on $F$.

### $R$-Algebraic Structure Homomorphism

Let $R$ be a ring.

Let $\struct {S, \ast_1, \ast_2, \ldots, \ast_n, \circ}_R$ and $\struct {T, \odot_1, \odot_2, \ldots, \odot_n, \otimes}_R$ be $R$-algebraic structures.

Let $\phi: S \to T$ be a mapping.

Then $\phi$ is an **$R$-algebraic structure homomorphism** if and only if:

- $(1): \quad \forall k \in \closedint 1 n: \forall x, y \in S: \map \phi {x \ast_k y} = \map \phi x \odot_k \map \phi y$
- $(2): \quad \forall x \in S: \forall \lambda \in R: \map \phi {\lambda \circ x} = \lambda \otimes \map \phi x$

where $\closedint 1 n = \set {1, 2, \ldots, n}$ denotes an integer interval.

Note that this definition also applies to modules and vector spaces.

### $G$-Module Homomorphism

Let $\struct {G, \cdot}$ be a group.

Let $\struct {V, \phi}$ and $\struct {W, \mu}$ be $G$-modules.

Then a linear transformation $f: V \to W$ is called a **$G$-module homomorphism** if and only if:

- $\forall g \in G: \forall v \in V: \map f {\map \phi {g, v} } = \map \mu {g, \map f v}$

### Homomorphism of Complexes

Let $\struct {R, +, \cdot}$ be a ring.

Let:

- $M: \quad \cdots \longrightarrow M_i \stackrel {d_i} {\longrightarrow} M_{i + 1} \stackrel {d_{i + 1} } {\longrightarrow} M_{i + 2} \stackrel {d_{i + 2} } {\longrightarrow} \cdots$

and

- $N: \quad \cdots \longrightarrow N_i \stackrel {d'_i} {\longrightarrow} N_{i + 1} \stackrel {d'_{i + 1} } {\longrightarrow} N_{i + 2} \stackrel {d'_{i + 2} } {\longrightarrow} \cdots$

be two differential complexes of $R$-modules.

Let $\phi = \set {\phi_i: i \in \Z}$ be a family of module homomorphisms $\phi_i: M_i \to N_i$.

Then $\phi$ is a **homomorphism of complexes** if and only if for each $i \in \Z$:

- $\phi_{i + 1} \circ d_i = \phi_i \circ d'_i$

## Homomorphic Image

As a homomorphism is a mapping, the **homomorphic image** of $\phi$ is defined in the same way as the image of a mapping:

- $\Img \phi = \set {t \in T: \exists s \in S: t = \map \phi s}$

## Homomorphism as Cartesian Product

Let $\phi: \struct {S, \circ} \to \struct {T, *}$ be a mapping from one algebraic structure $\struct {S, \circ}$ to another $\struct {T, *}$.

We define the cartesian product $\phi \times \phi: S \times S \to T \times T$ as:

- $\forall \tuple {x, y} \in S \times S: \map {\paren {\phi \times \phi} } {x, y} = \tuple {\map \phi x, \map \phi y}$

Hence we can state that $\phi$ is a homomorphism if and only if:

- $\map \ast {\map {\paren {\phi \times \phi} } {x, y} } = \map \phi {\map \circ {x, y} }$

using the notation $\map \circ {x, y}$ to denote the operation $x \circ y$.

The point of doing this is so we can illustrate what is going on in a commutative diagram:

- $\begin{xy} \xymatrix@L+2mu@+1em{ S \times S \ar[r]^*{\circ} \ar[d]_*{\phi \times \phi} & S \ar[d]^*{\phi} \\ T \times T \ar[r]_*{\ast} & T }\end{xy}$

Thus we see that $\phi$ is a homomorphism if and only if both of the composite mappings from $S \times S$ to $T$ have the same effect on all elements of $S \times S$.

## Also known as

Some sources refer to a **homomorphism** as a **morphism**, but this term is best reserved for its use in category theory.

## Also see

- Definition:Endomorphism: a homomorphism from a magma to itself

- Definition:Automorphism (Abstract Algebra): an isomorphism from a magma to itself.

- Results about
**homomorphisms**in the context of**abstract algebra**can be found**here**.

## Linguistic Note

The word **homomorphism** comes from the Greek **morphe** (* μορφή*) meaning

**form**or

**structure**, with the prefix

**homo-**meaning

**similar**.

Thus **homomorphism** means **similar structure**.

## Sources

- 1965: Seth Warner:
*Modern Algebra*... (previous) ... (next): Chapter $\text {II}$: New Structures from Old: $\S 12$: Homomorphisms - 1972: A.G. Howson:
*A Handbook of Terms used in Algebra and Analysis*... (previous) ... (next): $\S 7$: Homomorphisms and quotient algebras - 1998: David Nelson:
*The Penguin Dictionary of Mathematics*(2nd ed.) ... (previous) ... (next):**homomorphism** - 2008: David Nelson:
*The Penguin Dictionary of Mathematics*(4th ed.) ... (previous) ... (next):**homomorphism** - 2014: Christopher Clapham and James Nicholson:
*The Concise Oxford Dictionary of Mathematics*(5th ed.) ... (previous) ... (next):**homomorphism**