# nLab submodule

### Context

#### Algebra

higher algebra

universal algebra

# Contents

## Definition

Thoughout let $R$ be some ring. Write $R$Mod for the category of module over $R$. Write $U R Mod \to$ Set for the forgetful functor that sends a module to its underlying set.

###### Definition

For $N \in R Mod$ a module, a submodule of $N$ is a subset of $U(N)$ which

1. is a subgroup of the underlying group (closed under the addition in $N$);

2. is preserved by the $R$-action.

Equivalently this means:

###### Definition

A submodule of $N \in R Mod$ is a module homomorphism $i : K \to N$ whose underlying map $U(i)$ of sets is an injection.

And since the injections in $R$Mod are precisely the monomorphisms, this means that equivalently

###### Definition

A submodule of $N \in R Mod$ is a monomorphism $i : K \hookrightarrow N$ in $R$Mod. Hence a submodule is a subobject in $R$Mod.

###### Remark

Given a submodule $K \hookrightarrow N$, the quotient module $\frac{N}{K}$ is the quotient group of the underlying abelian groups.

## Examples

###### Example

For $N = R$ regarded as a module over itself, a submodule is precisely an ideal of $R$.

###### Example

For $f : N_1 \to N_2$ a homomorphism of modules,

1. the kernel $ker(f) \hookrightarrow N_1$ is a submodule of $N_1$,

2. the image $im(f) \hookrightarrow N_2$ is a submodule of $N_2$.

###### Remark

In example 2 quotient module of $N_2$ by the image is the cokernel of $f$

$coker(f) \simeq \frac{N_2}{im(f)} \,.$

## Properties

### Of free modules

Let $R$ be a ring.

###### Proposition

Assuming the axiom of choice, the following are equivalent

1. every submodule of a free module over $R$ is itself free;

2. every ideal in $R$ is a free $R$-module;

3. $R$ is a principal ideal domain.

A proof is in (Rotman, pages 650-651).

For instance