# Random-Walk Closeness Centrality

#### Definition

Consider a weighted network – either directed or undirected – with n nodes denoted by j=1, …, n; and a random walk process on this network with a transition matrix M. The element of M describes the probability of the random walker that has reached node i, proceeds directly to node j. These probabilities are defined in the following way.

where is the (i,j)th element of the weighting matrix A of the network. When there is no edge between two nodes, the corresponding element of the A matrix is zero.

The random walk closeness centrality of a node i is the inverse of the average mean first passage time to that node:

### Mean first passage time

The mean first passage time from node i to node j is the expected number of steps it takes for the process to reach node j from node i for the first time:

where P(i,j,r) denotes the probability that it takes exactly r steps to reach j from i for the first time. To calculate these probabilities of reaching a node for the first time in r steps, it is useful to regard the target node as an absorbing one, and introduce a transformation of M by deleting its j-th row and column and denoting it by . As the probability of a process starting at i and being in k after r-1 steps is simply given by the (i,k)th element of , P(i,j,r) can be expressed as

Substituting this into the expression for mean first passage time yields

Using the formula for the summation of geometric series for matrices yields

where I is the n-1 dimensional identity matrix.

For computational convenience, this expression can be vectorized as

where is the vector for first passage times for a walk ending at node j, and e is an n-1 dimensional vector of ones.

Mean first passage time is not symmetric, even for undirected graphs.

[Random walk closeness centrality. (2013, March 7). In Wikipedia]

#### Software

#### References

- NOH, J. D. & RIEGER, H. 2004. Random walks on complex networks. Physical review letters, 92, 118701.
- Random walk closeness centrality. (2013, March 7). In Wikipedia, The Free Encyclopedia. Retrieved 07:56, August 6, 2014, from http://en.wikipedia.org/w/index.php?title=Random_walk_closeness_centrality&oldid=542550793