The sigmoid activation function, 
, is a smooth function, similar to a stepwise function:
A sigmoid is a non-linear function, and a combination of sigmoids also produces a non-linear function. This allows us to combine neuron layers. A sigmoid activation function is not binary, which makes an activation with a set of values from the range [0,1], in contrast to a stepwise function. A smooth gradient also characterizes a sigmoid. In the range of values of from -2 to 2, the values, , change very quickly. This gradient ...
