At the most fundamental level, graphs are just entities and connections between them. Yet, the network topology gives rise to emergent properties. For instance, how information flows through a social network is partly a function who posts the message and how they are connected to the rest of the network, with their immediate connections being likely more important. In this section, I review three levels at which networks operate: local, mesoscale and global. They refer, respectively, to properties of the nodes, properties of parts of the network and properties of the whole network.

Local properties

Degree

In an undirected network, the degree of a vertex \(u\) (\(\deg u\)) refers to the number of edges that are incident on \(u\). In a directed network, this concept is split between indegree ([\(\deg^- u\)], the number of edges that have \(u\) as their destination) and outdegree ([\(\deg^+ u\)], number of edges that have \(u\) as their source). Weighted graphs extend this concept to weighted degree, in which \(\deg u = \sum_{i} w(e_{ui})\).

Local clustering coefficient

The (local) clustering coefficient of a vertex measures the probability that its neighbors are connected. It is computed as the ratio between number of triangles involving a vertex, and the number of triplets involving that same vertex.

Often, the clustering coefficient of a directed graph is computed without considering the direction of the edges.

Mesoscale properties

Modularity

The modularity measures how well a graph can be divided into modules. Given a partition of a graph into \(k\) modules, the modularity \(Q\) is computed as

\[Q = \sum_{i=1}^k (e_{ii} - {a_i^2})\]

where \(e_{ii} = \frac {\| \{\{u, v\} \mid u \in V_i, v \in V_i, \{u, v\} \in E \} \|} {\|E\|}\),\(a*i = \frac {\| \{\{u, v\} \mid u \in V_i, \{u, v\} \in E \} \|} {\|E\|}\) and \(V_i\) is the set of vertices in module \(i\). \(e*{ii}\) is the fraction of edges within module \(i\) and \(a_i\) is the fraction of edges incident with one vertex in module \(i\). \(Q\) will be large when the fraction of edges within the module is much larger than expected by chance.

Within-module degree

The within-module degree of a vertex is the module version of the degree. It is often normalized as a z-score; the z-score for node \(i\), mapped to module \(k\):

\[Z_i = \frac {\kappa_i - \bar \kappa_k} {\sigma_{\kappa_k}}\]

where \(\kappa_i\) is within-module degree (the number of edges between \(i\) and other vertices in module \(k\)); \(\bar \kappa_k\) is the average within-module degree; and \(\sigma_{\kappa_k}\) is the standard deviation of the within module degrees.

Global properties

Radius and diameter

The radius and the diameter measure how easy it is to traverse a graph. They both are quantities based on the maximum distance between any two vertices found in the graph. Specifically, the radius is the minimum maximum distance; the diameter is the maximum distance.

Global clustering coefficient

The global clustering coefficient of a graph is the ratio between closed and open triplets in that graph. Or, equivalently:

\[C = \frac {3 \times \text{triangles}} {\text{triplets}}\]

Often, the clustering coefficient of a directed graph is computed without considering the direction of the edges.

Centrality

Centrality assigns a score or a ranking to every vertex in the graph, which represents its importance in the network according to some metric. Degree and participation are examples of such metrics, but there are others.

WIP