Fig. 2.

An example of a tree to rules. At each intermediate node, an observation goes to the left branch if and only if the condition shown there is satisfied. A simple regression tree which can be represented as $y=20I(M_1<0)(M_2<1)+15I(M_1<0)I(M_2\ge 1)+10I(M_1\ge 0).$ Each leaf node defines a rule which can be expressed as a product of indicator functions of half spaces. Each rule specifies a ‘simple’ rectangular region in the input space