Exponential on Real Numbers is Group Isomorphism

Theorem
Let $\left({\R, +}\right)$ be the Additive Group of Real Numbers.

Let $\left({\R_+^*, \times}\right)$ be the Multiplicative Group of Positive Real Numbers.

Let $\exp: \left({\R, +}\right) \to \left({\R_+^*, \times}\right)$ be the mapping:
 * $x \mapsto \exp \left({x}\right)$

where $\exp$ is the exponential function.

Then $\exp$ is a group isomorphism.

Proof 1
From Exponent of Sum we have:
 * $\forall x, y \in \R: \exp \left({x + y}\right) = \exp x \cdot \exp y$

That is, $\exp$ is a group homomorphism.

Then we have that Exponential is Strictly Increasing and Convex.

From Strictly Monotone Function is Bijective, it follows that $\exp$ is a bijection.

So $\exp$ is a bijective group homomorphism, and so a group isomorphism.

Proof 2
First we confirm that:
 * The real numbers form a group under addition


 * The positive real numbers form a group under multiplication.

We have that $\exp$ is surjective since for all $y \in R^+$ there exists $x = \log(y) \in R$ which satisfies $\exp(x) = y$.

Also, $\exp$ is injective since it is strictly increasing.

Therefore, $\exp$ is a bijection.

Let $x, y \in R$.

Then:
 * $\exp(x + y) = \exp(x) \cdot \exp(y)$

from Exponent of Sum.

So $\exp$ is a homomorphism and a bijection and is therefore an isomorphism.