     -\frac{\pi}{2} &amp; \;\text{if $x=0,$ $y &lt; 0$}\\
                \text{undefined}&amp; \;\text{if $x=0,$ $y=0$}\\
                \end{array}\right.
                \end{eqnarray*}
            </div>
            Thus, if $z=r(\cos \Theta +i\sin \Theta),$ with $r&gt;0$ and $-\pi &lt; \Theta \leq \pi,$ then
            <div class="scroll-wrapper">
                \begin{eqnarray*}
                \textbf{arg}(z)= \{ \textbf{Arg}(z)+2n\pi \mid n \in  \mathbb{Z} \}.
                \end{eqnarray*}
    