An [[Imaginary Number]] is a [[Real Numbers|Real Number]] scaled by $i$ ($\sqrt{ -1 }$). $ \huge \C \setminus \R = \setbuild{ bi }{b\in \R} $ The notation for referring to the [[Imaginary Number|Imaginary]] part of a [[Complex Numbers|Complex Number]] is: $ \large \begin{align} \mathrm{Im}&: \C \to \C \setminus\R\\ \op{Im}&: a+bi \mapsto bi \end{align} $ Note that when using $\mathrm{Im}$ as a function, the convention is to use square brackets instead of parenthesis. >[!example] Example: $\mathrm{Im}[1+5i]=5i$