Domain of Real Natural Logarithm

Theorem
Let $\ln$ be the natural logarithm function on the real numbers.

Then the domain of $\ln$ is the set of strictly positive real numbers:
 * $\Dom \ln = \R_{>0}$

Proof
By definition of natural logarithm:


 * $\ln^{-1} = \exp$

From Exponential Tends to Zero and Infinity:
 * $\Dom \exp = \openint {-\infty} {+\infty}$
 * $\Img \exp = \openint 0 {+\infty}$

From Exponential is Strictly Increasing:
 * $\exp$ is strictly increasing.

From Strictly Monotone Real Function is Bijective, $\exp: \R \to \R_{>0}$ is a bijection.

Thus:
 * $\Dom \ln = \Img \exp$

and so $\Dom \ln = \R_{>0}$.