2 Data Mining Methods for Recommender Systems 55
output is obtained by summing up each of its input value according to the weights of its links and comparing its output against some threshold θk. The output function can be expressed using Eq. 2.7. The perceptron model is a linear classifier that has a simple and efficient learning algorithm. But, besides the simple Threshold Func- tion used in the Perceptron model, there are several other common choices for the activation function such as sigmoid, tanh, or step functions.