# Network Hubs

Some times real networks contain few nodes that are connected to a large portion of the nodes in the network. These nodes, often called ‘hubs’ (or global hubs), can change global properties of the network drastically, for example the length of the shortest path between two nodes can be significantly reduced by their presence.

The presence of hubs in real networks can be easily observed, for example, in flight networks airports such as Heathrow (UK) or Beijing capital IAP (China) have a very large number of incoming and outgoing flights in comparison to all other airports in the world. Now, if in addition to the network there is a partition of the nodes into different groups ‘local hubs’ can appear. For example, assume that the political division is a partition of the nodes (airports) into different countries. Then, some capital city airports can be local hubs as they have incoming and outgoing flights to most other airports in that same country. Note that a local hub might not be a global hub.

There are several ways to classify nodes based on different network properties. Take for example, hub nodes and non-hub nodes. One way to classify nodes as hub or non-hub uses the participation coefficient and the standardised within module degree (Gimera &  Amaral, 2005).

Consider a partition of the nodes into $latex N_M$ groups. Let $latex k_i$ be the degree of node $latex i$ and $latex k_{is}$ the number of links or edges to other nodes in the same group as node $latex i$. Then, the participation coefficient of node $latex i$ is:

$latex P_i = 1 – \sum_{s=1}^{N_M} k_{is}^2 / k_i^2$ .

Note that if node $latex i$ is connected only to nodes within its group then, the participation coefficient of node $latex i$ is 0. Otherwise if it is connected to nodes uniformly distributed across all groups then the participation coefficient is close to 1 (Gimera &  Amaral, 2005).

Now, the standardised within module degree:

$latex z_i= (k_i – \bar{k}_{s_i}) / \sigma_{k_{s_i}}$,

where $latex s_i$ is the group node $latex i$ belongs to and $latex \sigma_{k_{s_i}}$ is the standard deviation of $latex k$ in such group.

Gimera &  Amaral (2005) proposed a classification of the nodes of the network based on their corresponding values of the previous statistics. In particular they proposed a heuristic classification of the nodes depicted by the following plot

Image taken from the paper “Functional cartography of complex
metabolic networks” by Guimera and Amaral, 2005.

Guimera and Amaral (2005), named regions R1-R4 as non-hub regions and R5-R7 as hub regions. Nodes belonging to: R1 are labelled as ultra-peripheral nodes, R2 as peripheral nodes, R3 as nun-hub connector nodes, R4 as non-hub kinless nodes, R5 as provincial nodes, R6 as connector hubs and R7 as kinless hubs. For more details on this categorisation please see Guimera and Amaral (2005).

The previous regions give an intuitive classification of network nodes according to their connectivity under a given partition of the nodes. In particular it gives an easy way to differentiate hub nodes of non-hub nodes. However the classification of the nodes into these seven regions (R1-R7) depends on the initial partition of the nodes.

1. R. Guimerà, L.A.N. Amaral, Functional cartography of complex metabolic networks, Nature 433 (2005) 895–900