# The distribution of first hitting times of random walks on directed Erd\H{o}s-R\'enyi networks

## TL;DR

This paper analytically characterizes the distribution of first hitting times for random walks on directed Erdős-Rényi networks, revealing longer path lengths compared to undirected networks due to the absence of backtracking.

## Contribution

It provides explicit analytical expressions for the tail distribution of first hitting times and extends the approach to semi-ER networks with Poisson in-degree distributions.

## Key findings

- First hitting times are significantly longer in directed networks than undirected ones.
- The distribution of first hitting times is a product of exponential and Rayleigh distributions.
- Directed networks lack backtracking, leading to longer random walk paths.

## Abstract

We present analytical results for the distribution of first hitting times of random walkers (RWs) on directed Erd\H{o}s-R\'enyi (ER) networks. Starting from a random initial node, a random walker hops randomly along directed edges between adjacent nodes in the network. The path terminates either by the retracing scenario, when the walker enters a node which it has already visited before, or by the trapping scenario, when it becomes trapped in a dead-end node from which it cannot exit. The path length, namely the number of steps, $d$, pursued by the random walker from the initial node up to its termination, is called the first hitting time. Using recursion equations, we obtain analytical results for the tail distribution of first hitting times, $P(d>\ell)$. The distribution $P(d>\ell)$ can be expressed as a product of an exponential distribution and a Rayleigh distribution. We obtain expressions for the mean, median and standard deviation of this distribution in terms of the network size and its mean degree. We also calculate the distribution of last hitting times, namely the path lengths of self-avoiding walks on directed ER networks, which do not retrace their paths. The last hitting times are found to be much longer than the first hitting times. The results are compared to those obtained for undirected ER networks. It is found that the first hitting times of RWs in a directed ER network are much longer than in the corresponding undirected network. This is due to the fact that RWs on directed networks do not exhibit the backtracking scenario, which is a dominant termination mechanism of RWs on undirected networks. It is shown that our approach also applies to a broader class of networks, referred to as semi-ER networks, in which the distribution of in-degrees is Poisson, while the out-degrees may follow any desired distribution with the same mean as the in-degree distribution.