Note 1 to entry: The argument $\arg c$ of the complex number $c=a+jb=\left|c\right|e^{j\phi }$ is equal to:
$\arccos (a/\sqrt{a^2+b^2})$ if $b\ge 0$ and $-\arccos (a/\sqrt{a^2+b^2})$ if $b<0$, where $0\le \arccos x\le \pi$.
It is not defined if $a=b=0$.

Note 2 to entry: The concept of argument may be applied to complex scalar quantities.