## 2.2 Networks and graphs (Ch. 2.2)

The components in a network are called nodes or vertices and the direct connections between them are called links or edges. When we are describing a network in real life, it’s better to use networks, nodes, and links, whereas graphs, vertices, and edges are mostly used for an abstract representation of networks. Some people do not care about these distinctions, though.

N, meaning the number of nodes, is also called the size of the network. Nodes are labeled as i = 1,2,3,…, N

L, representing the total number of links in a network.

For an individual link, we may represent it as $$(i, j)$$. For example, (2,4) denotes the link connecting nodes 2 and 4.

Links can either be directed or undirected. If I make a phone call to you, then the link between us is from me to you. It is directed. If we are friends on social media, then this link is undirected.

A network is directed if all of its links are directed. If all of its links are undirected, then this network is an undirected network.