the set of nodes is simply N = {1, 2, 3, n}. The second set in a graph is a list of the links between these nodes. Thus, L is a set of links (i, j) where i # j and i and j are elements of the set of nodes N. For example, if the network has four nodes, N = {1, 2, 3, 4}, a list of the links might be L = {(1, 2), (1, 3), (1, 4), (3, 4)}. The graph with four nodes and these four links is shown in Figure 1.2. Even a simple graph with a few nodes has many possible configurations. The notation G refers to the set of nodes N and the set of links between those nodes L: G = (N, L). This provides a formal description of the basic structure of the network. As we will soon see, there are many other aspects of a network that must be identified. The description of links between nodes in a network can include specification of a direction. A picture of a network that specifies the direction of the connections The Structure and Functions of Networks Figure 1.3. A one-way network in which nodes 1 and 4 are initial nodes, and nodes 2, 3, and 4 are terminal nodes. in a network is called a directed graph. When the nodes have labels that describe their function, such a graph is called a flow graph - made popular by the widespread use of flow charts in engineering, computer science, and management. An example of a one-way network is a cable television system in which signals are transmitted to receivers. Also, a radio broadcast system constitutes a one-way network with the radio station and radio receivers as nodes, and the radio transmitter and a particular frequency on the radio spectrum as the links. A one-way network with four nodes and three links is shown in Figure 1.3. In the figure, nodes 1 and 4 are initial nodes, and nodes 2, 3, and 4 are terminal nodes. The links are shown as curved lines or arcs with arrows to indicate the direction of flow. A two-way network can be viewed as a directed network in which there are always paths in both directions. For example, a one-way network with matching links forms an equivalent two-way n