# Definition:Exponential Function/Real/Inverse of Natural Logarithm

## Definition

Consider the natural logarithm $\ln x$, which is defined on the open interval $\openint 0 {+\infty}$.

$\ln x$ is strictly increasing.
the inverse of $\ln x$ always exists.

The inverse of the natural logarithm function is called the exponential function, which is denoted as $\exp$.

Thus for $x \in \R$, we have:

$y = \exp x \iff x = \ln y$

The number $\exp x$ is called the exponential of $x$.

The domain of $\exp$ is $\R$, and the codomain of $\exp$ is $\R_{>0}$.