Pattern Recognition, 4th Edition

4.3. The Two-Layer Perceptron

To separate the two classes A and B in Figure 4.1, a first thought that comes to mind is to draw two, instead of one, straight lines.

Figure 4.4 shows two such possible lines, g¹(x) = g²(x) = 0, as well as the regions in space for which g¹(x) 0, g²(x) 0. The classes can now be separated. Class A is to the right (+) of g¹(x) and to the left (−) of g²(x). The region corresponding to class B lies either to the left or to the right of both lines. What we have really done is to attack the problem in two successive phases. ...