# Subset of Abelian Group Generated by Product of Element with Inverse Element is Subgroup/Examples/Non-Zero Integers in Non-Zero Reals under Multiplication

## Examples of Use of Subset of Abelian Group Generated by Product of Element with Inverse Element is Subgroup

Let $\struct {\Z_{\ne 0}, \times}$ denote the algebraic structure formed by the set of non-zero integers under multiplication.

Let $\struct {\R_{\ne 0}, \times}$ denote the algebraic structure formed by the set of non-zero real numbers under multiplication.

Let $H$ denote the set:

- $H := \set {x \times y^{-1}: x, y \in \Z_{\ne 0} }$

Then $H$ is the subgroup of $\struct {\R_{\ne 0}, \times}$ which is the set of non-zero rational numbers under multiplication:

- $H = \struct {\Q_{\ne 0}, \times}$

## Proof

From Non-Zero Integers Closed under Multiplication:

- $\forall a, b \in \Z_{\ne 0}: a \times b \in \Z_{\ne 0}$

From Subset of Abelian Group Generated by Product of Element with Inverse Element is Subgroup:

- $H$ is a subgroup of $\struct {\R_{\ne 0}, \times}$

The elements of $H$ are all the numbers of the form:

- $x = \dfrac p q$

where $p, q \in \Z_{\ne 0}$

and thus by definition rational numbers.

Hence the result.

We also note that Non-Zero Rational Numbers under Multiplication form Abelian Group for further confirmation.

$\blacksquare$

## Sources

- 1965: J.A. Green:
*Sets and Groups*... (previous) ... (next): Chapter $5$: Subgroups: Exercise $4$