# Non-Zero Real Numbers under Multiplication form Abelian Group/Proof 1

## Contents

## Theorem

Let $\R_{\ne 0}$ be the set of real numbers without zero:

- $\R_{\ne 0} = \R \setminus \set 0$

The structure $\struct {\R_{\ne 0}, \times}$ is an uncountable abelian group.

## Proof

Taking the group axioms in turn:

### G0: Closure

From Non-Zero Real Numbers Closed under Multiplication: Proof 2, $\R_{\ne 0}$ is closed under multiplication.

Note that proof 2 needs to be used specifically here, as proof 1 rests on this result.

$\Box$

### G1: Associativity

Real Multiplication is Associative.

$\Box$

### G2: Identity

The identity element of real number multiplication is the real number $1$:

- $\exists 1 \in \R: \forall a \in \R_{\ne 0}: a \times 1 = a = 1 \times a$

$\Box$

### G3: Inverses

Each element $x$ of the set of non-zero real numbers $\R_{\ne 0}$ has an inverse element $\dfrac 1 x$ under the operation of real number multiplication:

- $\forall x \in \R_{\ne 0}: \exists \dfrac 1 x \in \R_{\ne 0}: x \times \dfrac 1 x = 1 = \dfrac 1 x \times x$

$\Box$

### C: Commutativity

Real Multiplication is Commutative.

$\Box$

### Infinite

Real Numbers are Uncountably Infinite.

$\blacksquare$

## Sources

- 1965: J.A. Green:
*Sets and Groups*... (previous) ... (next): $\S 4.5$. Examples of groups: Example $82$

- 1974: Thomas W. Hungerford:
*Algebra*... (previous) ... (next): $\text{I}$: Groups: $\S 1$: Semigroups, Monoids and Groups