$\left\{\begin{matrix}\\A=-\sqrt{C}\text{; }A=\sqrt{C}\text{, }&\text{unconditionally}\\A\in \mathrm{C}\text{, }&C=0\end{matrix}\right.$

$\left\{\begin{matrix}A\in \mathrm{R}\text{, }&C=0\\A=-\sqrt{C}\text{; }A=\sqrt{C}\text{, }&C>0\end{matrix}\right.$

$C=0$
$C=A^{2}$