$$\left. \begin{array} { l } { [ a \cdot b ) = a \lt a \lt b } \\ { ( a \cdot b ) = b \lt x \lt b } \end{array} \right.$$
$a=1\text{, }b\in \mathrm{C}$
$a\in \mathrm{C}\text{, }b=0$
$a=1\text{, }b\in \mathrm{R}$
$a\in \mathrm{R}\text{, }b=0$
