# Alpha centrality

In graph theory and social network analysis, alpha centrality is a measure of centrality of nodes within a graph. It is an adaptation of eigenvector centrality with the addition that nodes are imbued with importance from external sources.

## Definition

Given a graph with adjacency matrix $A_{i,j}$ the alpha centrality is defined as follows:

$\vec{x} = (I-\alpha A^T)^{-1}\vec{e} \,$

where $e_j$ is the external importance given to node $j$, and $\alpha$ is a parameter.[1]

## Motivation

To understand alpha centrality one must first understand Eigenvector Centrality. An intuitive process to compute eigenvector centrality is to give every node a starting random positive amount of influence. Each node then splits its influence evenly and divides it amongst its outward neighbors, receiving from its inward neighbors in kind. This process repeats until everyone is giving out as much as they're taking in and the system has reached steady state. The amount of influence they have at this steady state is their eigenvector centrality. Computationally this process is called the power method. We know that this process has converged when the vector of influence changes only by a constant as follows.

$x_i = \frac{1}{\lambda} A^T_{i,j}x_j$

Where $x_i$ is the amount of influence that node $i$ carries, $A_{i,j}$ is the adjacency matrix and $\lambda$ happens to be the principal eigenvalue (although this is not terribly important here).

Alpha centrality enhances this process by allowing nodes to have external sources of influence. The amount of influence that node $i$ receives at every round is encoded in $e_i$. The process described above should now stop when

$x_i = \alpha A^T_{i,j}x_j + e_i \,$

Where $\alpha$ is a constant that trades off the importance of external influence against the importance of connection. When $\alpha=0$ only the external influence matters. When $\alpha$ is very large then only the connectivity matters, i.e. we reduce to the eigenvector centrality case.

Rather than perform the iteration described above we can solve this system for $x$, obtaining the following equation:

$x = (I-\alpha A^T)^{-1}e \,$

## Applications

Alpha centrality is implemented in igraph library for network analysis and visualization.[2]