# A Module is Irreducible if and only if It is a Cyclic Module With Any Nonzero Element as Generator

## Problem 434

Let $R$ be a ring with $1$.
A nonzero $R$-module $M$ is called irreducible if $0$ and $M$ are the only submodules of $M$.
(It is also called a simple module.)

(a) 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.

(b) Determine all the irreducible $\Z$-modules.

## Proof.

### (a) 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.

$(\implies)$ Suppose that $M$ is an irreducible module.
Let $a\in M$ be any nonzero element and consider the submodule $(a)$ generated by the element $a$.

Since $a$ is a nonzero element, the submodule $(a)$ is non-zero. Since $M$ is irreducible, this yields that
$M=(a).$ Hence $M$ is a cyclic module generated by $a$. Since $a$ is any nonzero element, we conclude that the module $M$ is a cyclic module with any nonzero element as its generator.

$(\impliedby)$ Suppose that $M$ is a cyclic module with any nonzero element as its generator.
Let $N$ be a nonzero submodule of $M$. Since $N$ is non-zero, we can pick a nonzero element $a\in N$. By assumption, the non-zero element $a$ generates the module $M$.

Thus we have
$(a) \subset N \subset M=(a).$ It follows that $N=M$, and hence $M$ is irreducible.

### (b) Determine all the irreducible $\Z$-modules.

By the result of part (a), any irreducible $\Z$-module is generated by any nonzero element.
We first claim that $M$ cannot contain an element of infinite order. Suppose on the contrary $a\in M$ has infinite order.

Then since $M$ is irreducible, we have
$M=(a)\cong \Z.$ Since $\Z$-module $\Z$ has, for example, a proper submodule $2\Z$, it is not irreducible. Thus, the module $M$ is not irreducible, a contradiction.

It follows that any irreducible $\Z$-module is a finite cyclic group.
(Recall that any $\Z$-module is an abelian group.)
We claim that its order must be a prime number.

Suppose that $M=\Zmod{n}$, where $n=ml$ with $m,l > 1$.
Then
$(\,\bar{l}\,)=\{l+n\Z, 2l+n\Z, \dots, (m-1)l+n\Z\}$ is a proper submodule of $M$, and it is a contradiction.
Thus, $n$ must be prime.

We conclude that any irreducible $\Z$-module is a cyclic group of prime order.

## Related Question.

Here is another problem about irreducible modules.

Problem. 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.

For a proof of this problem, see the post “A Module $M$ is Irreducible if and only if $M$ is isomorphic to $R/I$ for a Maximal Ideal $I$.“.

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1. 06/10/2017

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