# Exponential of Real Number is Strictly Positive/Proof 2

## Theorem

Let $x$ be a real number.

Let $\exp$ denote the (real) exponential function.

Then:

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

## Proof

This proof assumes the limit definition of $\exp$.

That is, let:

$\displaystyle \exp x = \lim_{n \mathop \to \infty} \map {f_n} x$

where $\map {f_n} x = \paren {1 + \dfrac x n}^n$

First, fix $x \in \R$.

Let $N = \ceiling {\size x}$, where $\ceiling {\, \cdot \,}$ denotes the ceiling function.

Then:

 $\ds \exp x$ $=$ $\ds \lim_{n \mathop \to \infty} \map {f_n} x$ $\ds$ $=$ $\ds \lim_{n \mathop \to \infty} \map {f_{n + N} } x$ Tail of Convergent Sequence $\ds$ $\ge$ $\ds \map {f_{n + N} } x$ Exponential Sequence is Eventually Increasing and Limit of Bounded Convergent Sequence is Bounded $\ds$ $>$ $\ds 0$ Corollary to Exponential Sequence is Eventually Increasing

$\blacksquare$