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Bayesian Reasoning and Machine Learning

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CHAPTER 2<br />

Basic Graph Concepts<br />

Often we have good reason to believe that one event affects another, or conversely that some events are<br />

independent. Incorporating such knowledge can produce models that are better specified <strong>and</strong> computationally<br />

more efficient. Graphs describe how objects are linked <strong>and</strong> provide a convenient picture for describing<br />

related objects. We will ultimately introduce a graph structure among the variables of a probabilistic model<br />

to produce a ‘graphical model’ that captures relations among the variables as well as their uncertainties. In<br />

this chapter, we introduce the required basic concepts from graph theory.<br />

2.1 Graphs<br />

Definition 2.1 (Graph). A graph G consists of nodes (also called vertices) <strong>and</strong> edges (also called links)<br />

between the nodes. Edges may be directed (they have an arrow in a single direction) or undirected. Edges<br />

can also have associated weights. A graph with all edges directed is called a directed graph, <strong>and</strong> one with<br />

all edges undirected is called an undirected graph.<br />

A B<br />

D<br />

C<br />

A B<br />

D<br />

C<br />

E An directed graph G consists of directed edges between nodes.<br />

E An undirected graph G consists of undirected edges between nodes.<br />

Graphs with edge weights are often used to model networks <strong>and</strong> flows along ‘pipes’, or distances between<br />

cities, where each node represents a city. We will also make use of these concepts in chapter(5) <strong>and</strong> chapter(28).<br />

Our main use of graphs though will be to endow them with a probabilistic interpretation <strong>and</strong> we<br />

develop a connection between directed graphs <strong>and</strong> probability in chapter(3). Undirected graphs also play a<br />

central role in modelling <strong>and</strong> reasoning with uncertainty. Essentially, two variables will be independent if<br />

they are not linked by a path on the graph. We will discuss this in more detail when we consider Markov<br />

networks in chapter(4).<br />

Definition 2.2 (Path, ancestors, descendants). A path A ↦→ B from node A to node B is a sequence of<br />

nodes that connects A to B. That is, a path is of the form A0, A1, . . . , An−1, An, with A0 = A <strong>and</strong> An = B<br />

25

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