# Exponential of Real Number is Strictly Positive/Proof 5/Lemma

## Theorem

Let $x$ be a real number.

Let $\exp$ denote the (real) Exponential Function.

Then:

$\forall x \in \R: \exp x \ne 0$

## Proof

This proof assumes the definition of $\exp$ as the solution to an initial value problem.

That is, suppose $\exp$ satisfies:

$(1): \quad D_x \exp x = \exp x$
$(2): \quad \exp \left({0}\right) = 1$

on $\R$.

Aiming for a contradiction, suppose that $\exists \alpha \in \R: \exp \alpha = 0$.

Suppose that $\alpha > 0$.

Let $J = \left[{0 \,.\,.\, \alpha}\right]$.

From Exponential Function is Continuous, $\exp$ is continuous on $J$.

$\exists K \in \R: \forall x \in J: \left\vert{\exp x}\right\vert < K$

Then, $\forall n \in \N : \exists c_n \in J$ such that:

 $\displaystyle 1$ $=$ $\displaystyle \exp 0$ $\quad$ By hypothesis $(2)$ $\quad$ $\displaystyle$ $=$ $\displaystyle \sum_{j \mathop = 0}^{n-1} \frac{\exp^{j} \left({\alpha}\right)}{j!} \left({-\alpha}\right)^{j} + \frac{\exp c_n}{n!} \left({-\alpha}\right)^{n}$ $\quad$ Taylor's Theorem for Univariate Functions $\quad$ $\displaystyle$ $=$ $\displaystyle \sum_{j \mathop = 0}^{n-1} \frac{\exp \left({\alpha}\right)}{j!} \left({-\alpha}\right)^{j} + \frac{\exp c_n}{n!} \left({-\alpha}\right)^{n}$ $\quad$ By hypothesis $(1)$ $\quad$ $\displaystyle$ $=$ $\displaystyle \sum_{j \mathop = 0}^{n-1} \frac 0 {j!} \left({-\alpha}\right)^{j} + \frac{\exp c_n}{n!} \left({-\alpha}\right)^{n}$ $\quad$ From our assumption aiming at a contradiction $\quad$ $\displaystyle$ $=$ $\displaystyle \frac{\exp c_n}{n!} \left({-\alpha}\right)^{n}$ $\quad$ $\quad$

So $\forall n \in \N :1 \le K \dfrac{\alpha^{n}}{n!}$

That is, dividing both sides by $K$:

$\forall n \in \N: \dfrac 1 K \le \dfrac{\alpha^{n}}{n!}$

But from Power over Factorial, $\dfrac{\alpha^{n}}{n!} \to 0$.

The same argument, mutatis mutandis proves the result for $\alpha < 0$.
By hypothesis $(2)$:
 $\displaystyle \alpha = 0 \ \$ $\displaystyle \implies \ \$ $\displaystyle \exp \alpha$ $=$ $\displaystyle 1$ $\quad$ $\quad$ $\displaystyle$ $\ne$ $\displaystyle 0$ $\quad$ $\quad$
$\blacksquare$