Should you write $i = \sqrt{-1}$ or $i := \sqrt{-1}$?

Question

Is one of the following more correct than the other, or does it simply not matter?

$i = \sqrt{-1} \quad$ or $\quad i := \sqrt{-1}$?

Here I am using $:=$ to mean "defined to be equal to."

Answer 1

None of them are correct as the square root is not a single-valued function. The best expression (i.e. simplest) defining $i$ would be $i^2=-1$.