sigmoid

\[\sigma(x) = \frac{1}{1+e^{-x}} = \frac{e^x}{e^x + 1}\]

Things to note:

• \(1 - \sigma(x) = 1 - \frac{1}{1+e^{-x}} = \frac{e^{-x}}{1+e^{-x}} = \frac{1}{e^{x} + 1} = \sigma(-x)\)