# Additive Group and Multiplicative Group of Field are not Isomorphic

## Theorem

Let $\struct {F, +, \times}$ be a field whose zero is $0_F$ and whose unity is $1_F$.

Let $\struct {F, +}$ denote the additive group of $F$.

Let $\struct {F_{\ne 0_F}, \times}$ denote the multiplicative group of $F$.

Then $\struct {F, +}$ and $\struct {F_{\ne 0_F}, \times}$ are not isomorphic to each other.

## Proof

Aiming for a contradiction, suppose $\phi: \struct {F_{\ne 0_F}, \times} \to \struct {F, +}$ is an isomorphism.

By definition:

- $0_F$ is the identity of $\struct {F, +}$

and

- $1_F$ is the identity of $\struct {F_{\ne 0_F}, \times}$.

We have that:

\(\ds 0_F\) | \(=\) | \(\ds \map \phi {1_F}\) | Epimorphism Preserves Identity | |||||||||||

\(\ds \) | \(=\) | \(\ds \map \phi {\paren {-1_F} \times \paren {-1_F} }\) | ||||||||||||

\(\ds \) | \(=\) | \(\ds \map \phi {-1_F} + \map \phi {-1_F}\) | Definition of Group Isomorphism |

and so by definition $F$ has characteristic $2$.

Let $x \in \struct {F_{\ne 0_F}, \times}$.

Then:

\(\ds \map \phi {x^2}\) | \(=\) | \(\ds \map \phi x + \map \phi x\) | Definition of Group Isomorphism | |||||||||||

\(\ds \) | \(=\) | \(\ds 0_F\) | as $F$ has characteristic $2$ |

As $\phi$ is an isomorphism, it is also a monomorphism.

From Kernel is Trivial iff Monomorphism:

- $\map \ker \phi = \set {1_F}$

and so $x^2 = 1$.

Thus $x = 1$ and so $\order F = 2$.

Thus $\order {F_{\ne 0_F} } = 1$.

So $\phi$ is a bijection from a set of cardinality $1$ to a set of cardinality $2$.

So $\phi$ cannot be a bijection and so cannot be an isomorphism.

Hence the result by Proof by Contradiction.

$\blacksquare$

## Sources

- 1978: Thomas A. Whitelaw:
*An Introduction to Abstract Algebra*... (previous) ... (next): Chapter $9$: Rings: Exercise $16$