# Automorphism group of the group $(\mathbb{R}^n,+)$

As a vector space, it is obvious and well-known that the automorphism group of $\mathbb{R}^n$ is $GL_n(\mathbb{R})$.

My question is: What is the $\text{Aut}((\mathbb{R}^n,+))$ ($n \ge 2$) as a group? Clearly, $\text{Aut}((\mathbb{R}^n,+))$ contains $GL_n(\mathbb{R})$.

Given that the automorphism group of $\mathbb{R}$ is enormous, to simplify matters, I would greatly appreciate an answer with respect to continuous automorphisms (if this is known).

The question above is related to this question concerning additive maps that do not preserve scalar multiplication. In particular, are there continuous isomorphisms of $\mathbb{R}^n$ ($n \ge 2$) that do not preserve scalar multiplication? (Clearly, this would indicate that the containment above is strict.)

• By unique divisibility of $\Bbb{R}^n$ any endomorphism of abelian groups is $\Bbb{Q}$-linear. Because $\Bbb{Q}$ is dense in $\Bbb{R}$ won't this quickly lead to the conclusion that a continuous endomorphism of abelian groups is necessarily $\Bbb{R}$-linear? – Jyrki Lahtonen Nov 17 '17 at 22:26
• There are noncontinuous maps that satisfy the desired property. I agree with jyrki's comment once you require continuity. – Andres Mejia Nov 17 '17 at 22:40

## 2 Answers

Continuous homomorphisms are adressed in @JyrkiLahtonen's comment, so I'll adress non-continuous ones.

Note that $\Bbb R$ is a $\mathfrak{c}$-dimensional $\Bbb Q$ vector space. ($\mathfrak{c}$ is the cardinality of $\Bbb R$)

So as $\Bbb Q$-vector spaces and in particular as groups we have:

$\Bbb R^n \cong \left(\Bbb Q^{(\mathfrak{c})}\right)^n \cong \Bbb Q^{(n\mathfrak{c})} \cong \Bbb{Q}^{(\mathfrak{c})} \cong \Bbb R$ (This isomorphism is not continuous!)

Thus $\operatorname{Aut}(\Bbb R^n) \cong \operatorname{Aut}(\Bbb R)$, which is adressed in the linked wikipedia page.

Note that this also illustrates how large $\operatorname{Aut}(\Bbb R)$ is: it contains a copy of $\operatorname{GL}_n(\Bbb R)$ for every $n\in \Bbb N$.

The set of continuous automorphisms of $\mathbb R^n$ is $\mathrm{GL}_n(\mathbb R)$.

To see this assume $f\colon\mathbb R^n \to \mathbb R^n$ is an additive homomorphism, i.e. $f(u + v) = f(u) + f(v)$ and $f(0) = 0$, but we know nothing about whether it respects scalar multiplication. If $n \geq 0$ is an integer then $f(nu) = nf(u)$ because multiplication by $n$ is repeated addition and $f$ respects addition. If $n \leq 0$ is an integer then $f(nu) = -f(-nu) = -(-n)f(u) = nf(u)$. So $f$ respects scalars from $\mathbb Z$. Now $nf\left(\frac{1}{n}u\right) = f\left(n\frac{1}{n}u\right) = f(u)$ so multiply both sides by $\frac{1}{n}$ to get $f\left(\frac{1}{n}u\right) = \frac{1}{n}f(u)$. Since $f$ already respects $\mathbb Z$ we get now that $f$ is $\mathbb Q$-linear.

None of this used continuous so far. The automorphism group of $\mathbb R^n$ is $\mathrm{End}_{\mathbb Q}(\mathbb R^n)$. If we ask that $f$ be continuous and $r$ is a real number then take rationals $a_i$ such that $a_i \to r$ as $i \to \infty$. We have $a_if(u) = f(a_iu)$. Taking the limit as $i \to \infty$ on both sides (and using that $f$ is continuous to pass this limit to the inside of $f$) we get $rf(u) = f(ru)$. So if $f$ is continuous then $f$ is $\mathbb R$-linear.