# Equivalence of Definitions of Commutative Local Ring

## Theorem

Let $A$ be a commutative ring with unity.

The following definitions of the concept of **Commutative Local Ring** are equivalent:

### Definition 1

The ring $A$ is **local** if and only if it has a unique maximal ideal.

### Definition 2

The ring $A$ is **local** if and only if it is nontrivial and the sum of any two non-units is a non-unit.

### Definition 3

Let $M \subseteq A$ be the subset of the non-units of $A$.

The ring $A$ is **local** if and only if $M$ is a proper ideal of $A$.

## Proof

### Definition 1 implies Definition 2

Let $\mathfrak m \subsetneq A$ be the unique maximal ideal.

First, $A$ is nontrivial, since $1 \notin \mathfrak m$.

Secondly, let $x, y \in A$ be non-units.

Let $\ideal x$ and $\ideal y$ be the principal ideals generated by $x$ and $y$, respectively.

In view of the unique maximality, $\ideal x \subseteq \mathfrak m$ and $\ideal y \subseteq \mathfrak m$.

In particular:

- $x, y \in \mathfrak m$

which implies:

- $x + y \in \mathfrak m$

Therefore $x + y$ is a non-unit.

$\Box$

### Definition 2 implies Definition 3

First, by the assumption:

- $x, y \in M \implies x + y \in M$

Secondly, let $a \in A$ and $x \in M$.

We shall show that $a x \in M$.

Aiming for a contradiction, suppose there exists a $u \in A$ such that:

- $u \paren {a x} = 1$

Thus:

- $\paren {u a} x = 1$

which means that $x$ is a unit.

This contradicts the fact that $x \in M$.

Therefore $a x \in M$.

Altogether $M$ is an ideal.

Finally, $1 \notin M$, since $A$ is nontrivial.

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Therefore $M$ is a proper ideal.

$\Box$

### Definition 3 implies Definition 1

Let $I$ be an arbitrary ideal such that $M \subsetneq I$.

Let $u \in I \setminus M$.

Then $u$ is a unit by the definition of $M$.

Therefore $M = A$.

Furthermore, $M$ is assumed to be a proper ideal.

Therefore $M$ is a maximal ideal.

On the other hand, let $N$ be an arbitrary maximal ideal.

As $N$ does not contain a unit:

- $N \subseteq M$

which implies:

- $N = M$

Therefore $M$ is the unique maximal ideal.

$\blacksquare$