$\textbf{arg} (z)$ is a set rather than a number. In
                particular,
                the relation $$\textbf{arg} (z_1) = \textbf{arg} (z_2)$$ is not an equation, but expresses equality
                of two sets.</p>

            <p>As a consequence, two non-zero complex numbers $r_1 (\cos \varphi_1 + i \sin\varphi_1)$ and $r_2
                (\cos \varphi_2 + i \sin\varphi_2)$
                are equal if and only if
            </p><div class="scroll-wrapper">
                $$r_1=r_2,\quad