Definition:Natural Logarithm/Complex/Definition 1

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Let $z = r e^{i \theta}$ be a complex number expressed in exponential form such that $z \ne 0$.

The complex natural logarithm of $z \in \C_{\ne 0}$ is the multifunction defined as:

$\map \ln z := \set {\map \ln r + i \paren {\theta + 2 k \pi}: k \in \Z}$

where $\map \ln r$ is the natural logarithm of the (strictly) positive real number $r$.

Also defined as

It can also be written:

$\map \ln z := \ln \cmod z + i \arg z$


$\cmod z$ is the modulus of $z$
$\arg z$ is the argument of $z$, which is a multifunction.

Principal Branch

The principal branch of the complex natural logarithm is usually defined in one of two ways:

\(\ds \map \Ln z\) \(=\) \(\ds \map \ln r + i \theta\) for $\theta \in \hointr 0 {2 \pi}$
\(\ds \map \Ln z\) \(=\) \(\ds \map \ln r + i \theta\) for $\theta \in \hointl {-\pi} \pi$

It is important to specify which is in force during a particular exposition.


The notation for the natural logarithm function is misleadingly inconsistent throughout the literature. It is written variously as:

$\ln z$
$\log z$
$\Log z$
$\log_e z$

The first of these is commonly encountered, and is the preferred form on $\mathsf{Pr} \infty \mathsf{fWiki}$. However, many who consider themselves serious mathematicians believe this notation to be unsophisticated.

The second and third are ambiguous (it doesn't tell you which base it is the logarithm of).

While the fourth option is more verbose than the others, there is no confusion about exactly what is meant.

Also see

  • Results about logarithms can be found here.

Linguistic Note

The word logarithm comes from the Ancient Greek λόγος (lógos), meaning word or reason, and ἀριθμός (arithmós), meaning number.