# Definition:Isomorphism (Abstract Algebra)

*This page is about isomorphisms in abstract algebra. For other uses, see Definition:Isomorphism.*

## Contents

## Definition

An **isomorphism** is a homomorphism which is a bijection.

That is, it is a mapping which is both a monomorphism and an epimorphism.

An algebraic structure $\left({S, \circ}\right)$ is **isomorphic to** another algebraic structure $\left({T, *}\right)$ iff there exists an isomorphism from $\left({S, \circ}\right)$ to $\left({T, *}\right)$, and we can write $S \cong T$ (although notation may vary).

### Semigroup Isomorphism

Let $\left({S, \circ}\right)$ and $\left({T, *}\right)$ be semigroups.

Let $\phi: S \to T$ be a (semigroup) homomorphism.

Then $\phi$ is a semigroup isomorphism iff $\phi$ is a bijection.

### Monoid Isomorphism

Let $\left({S, \circ}\right)$ and $\left({T, *}\right)$ be monoids.

Let $\phi: S \to T$ be a (monoid) homomorphism.

Then $\phi$ is a monoid isomorphism if and only if $\phi$ is a bijection.

### Group Isomorphism

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

Let $\phi: G \to H$ be a (group) homomorphism.

Then $\phi$ is a group isomorphism if and only if $\phi$ is a bijection.

### Ring Isomorphism

Let $\left({R, +, \circ}\right)$ and $\left({S, \oplus, *}\right)$ be rings.

Let $\phi: R \to S$ be a (ring) homomorphism.

Then $\phi$ is a ring isomorphism if and only if $\phi$ is a bijection.

### $F$-Isomorphism

Let $R, S$ be rings with unity.

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

Let $\phi: R \to S$ be an $F$-homomorphism such that $\phi$ is bijective.

Then $\phi$ is an **$F$-isomorphism**.

The relationship between $R$ and $S$ is denoted $R \cong_F S$.

### Field Isomorphism

Let $\left({F, +, \circ}\right)$ and $\left({K, \oplus, *}\right)$ be fields.

Let $\phi: F \to K$ be a (field) homomorphism.

Then $\phi$ is a field isomorphism if and only if $\phi$ is a bijection.

### R-Algebraic Structure Isomorphism

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

Let $\phi: S \to T$ be an $R$-algebraic structure homomorphism.

Then $\phi$ is an $R$-algebraic structure isomorphism iff $\phi$ is a bijection.

### Field Isomorphism

Let $\left({F, +, \circ}\right)$ and $\left({K, \oplus, *}\right)$ be fields.

Let $\phi: F \to K$ be a (field) homomorphism.

Then $\phi$ is a field isomorphism if and only if $\phi$ is a bijection.

### Ordered Structure Isomorphism

An **ordered structure isomorphism** from an ordered structure $\left({S, \circ, \preceq}\right)$ to another $\left({T, *, \preccurlyeq}\right)$ is a mapping $\phi: S \to T$ that is both:

- $(1): \quad$ An isomorphism, i.e. a bijective homomorphism, from the structure $\left({S, \circ}\right)$ to the structure $\left({T, *}\right)$
- $(2): \quad$ An order isomorphism from the ordered set $\left({S, \preceq}\right)$ to the ordered set $\left({T, \preccurlyeq}\right)$.

## Isomorphic Copy

Let $\phi: S \to T$ be an isomorphism.

Let $x \in S$.

Then $\map \phi x \in T$ is known as **the isomorphic copy of $x$ (under $\phi$)**.

## Also see

## Linguistic Note

The word **isomorphism** derives from the Greek **morphe** (* μορφή*) meaning

**form**or

**structure**, with the prefix

**iso-**meaning

**equal**.

Thus **isomorphism** means **equal structure**.

## Sources

- 1965: Seth Warner:
*Modern Algebra*... (previous) ... (next): $\S 6$ - 1972: A.G. Howson:
*A Handbook of Terms used in Algebra and Analysis*... (previous) ... (next): $\S 7$: Homomorphisms and quotient algebras