# Tagged: unit

## Problem 526

A ring is called local if it has a unique maximal ideal.

(a) Prove that a ring $R$ with $1$ is local if and only if the set of non-unit elements of $R$ is an ideal of $R$.

(b) Let $R$ be a ring with $1$ and suppose that $M$ is a maximal ideal of $R$.
Prove that if every element of $1+M$ is a unit, then $R$ is a local ring.

## Problem 494

Prove that the rings $\Z[x]$ and $\Q[x]$ are not isomoprhic.

## Problem 204

Is there a (not necessarily commutative) ring $R$ with $1$ such that the equation
$x+x=1$ has more than one solutions $x\in R$?

## Problem 192

Show that any finite integral domain $R$ is a field.

## Problem 188

Denote by $i$ the square root of $-1$.
Let
$R=\Z[i]=\{a+ib \mid a, b \in \Z \}$ be the ring of Gaussian integers.
We define the norm $N:\Z[i] \to \Z$ by sending $\alpha=a+ib$ to
$N(\alpha)=\alpha \bar{\alpha}=a^2+b^2.$

Here $\bar{\alpha}$ is the complex conjugate of $\alpha$.
Then show that an element $\alpha \in R$ is a unit if and only if the norm $N(\alpha)=\pm 1$.
Also, determine all the units of the ring $R=\Z[i]$ of Gaussian integers.

## Problem 172

Let $R$ be a commutative ring.

Then prove that $R$ is a field if and only if $\{0\}$ is a maximal ideal of $R$.

## Nilpotent Element a in a Ring and Unit Element $1-ab$
Let $R$ be a commutative ring with $1 \neq 0$.
An element $a\in R$ is called nilpotent if $a^n=0$ for some positive integer $n$.
Then prove that if $a$ is a nilpotent element of $R$, then $1-ab$ is a unit for all $b \in R$.