# A Module $M$ is Irreducible if and only if $M$ is isomorphic to $R/I$ for a Maximal Ideal $I$.

## Problem 449

Let $R$ be a commutative ring with $1$ and let $M$ be an $R$-module.

Prove that the $R$-module $M$ is irreducible if and only if $M$ is isomorphic to $R/I$, where $I$ is a maximal ideal of $R$, as an $R$-module.

## Definition (Irreducible module).

An $R$-module $M$ is called **irreducible** if $M$ is not the zero module and $0$ and $M$ are the only submodules of $M$.

An irreducible module

is also called a **simple** module.

## Proof.

$(\implies)$ Suppose that $M$ is an irreducible $R$-module.

Then by definition of an irreducible module, $M$ is not the zero module.

Take any nonzero element $m\in M$, and consider the cyclic submodule $(m)=Rm$ generated by $m$.

Since $M$ is irreducible, we must have $M=Rm$.

Now we define a map $f:R\to M$ by sending $r\in R$ to $f(r)=rm$.

Then the map $f$ is an $R$-module homomorphism regarding $R$ is an $R$-module.

In fact, we have

\begin{align*}

&f(r+s)=(r+s)m=rm+sm=f(r)+f(s)\\

&f(rs)=(rs)m=r(sm)=rf(s)

\end{align*}

for any $r, s\in R$.

Since $M=Rm$, the homomorphism $f$ is surjective.

Thus, by the first isomorphism theorem, we obtain

\[R/I\cong M,\]
where $I=\ker(f)$.

It remains to show that $I$ is a maximal ideal of $R$.

Suppose that $J$ is an ideal such that $I\subset J \subset R$.

Then by the third isomorphism theorem for rings, we know that $J/I$ is an ideal of the ring $R/I\cong M$, hence $J/I$ is a submodule.

Since $M$ is irreducible, we must have either $J/I=0$ or $J/I=M$.

This implies that $J=I$ or $J=M$.

Hence $I$ is a maximal ideal.

$(\impliedby)$ Suppose now that $M\cong R/I$ for some maximal ideal $I$ of $R$.

Let $N$ be any submodule of $R/I$. (We identified $M$ and $R/I$ by the above isomorphism.)

Then $N$ is an ideal of $R/I$ since $N$ is an abelian group and closed under the action of $R$, hence that of $R/I$.

Since $R$ is a commutative ring and $I$ is a maximal ideal of $R$, we know that $R/I$ is a field.

Thus, only the ideals of $R/I$ are $0$ or $R/I$.

Hence we have $N=0$ or $N=R/I=M$.

This proves that $M$ is irreducible.

## Related Question.

A similar technique in the proof above can be used to solve the following problem.

**Problem**. Let $R$ be a ring with $1$.

Prove that a nonzero $R$-module $M$ is irreducible if and only if $M$ is a cyclic module with any nonzero element as its generator.

See the post “A module is irreducible if and only if it is a cyclic module with any nonzero element as generator” for a proof.

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