Why do so many formula use Euler's number? What's the role of the "e" about in many formula?
I know that it's 2.7xxxx value and something good for compound interest. Please tell me how I think it as, when faced with in some formula.
$ex \begin{eqnarray}\sigma(z) \equiv\frac{1}{1+e^{-z}}\tag{3}\end{eqnarray}$