A sigmoid activation function is used in many neural networks. The properties of a sigmoid function mean that it shares some of the non-lonearity of a threshold function, but is differentiable enabling learning through the backpropagation algorithm.
Used on Chap. 6: pages 115, 116; Chap. 7: page 146; Chap. 9: page 192; Chap. 21: page 523
Sigmoid function smoothly limits extreme values