The joint node degree distribution in the Erdős-Rényi network
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The Erdős-Rényi random graph is the simplest model for node degree distribution, and it is one of the most widely studied. In this model, pairs of n vertices are selected and connected uniformly at random with probability p, consequently, the degrees for a given vertex follow the binomial distribution. If the number of vertices is large, the binomial can be approximated by Normal using the Central Limit Theorem, which is often allowed when min (np, n(1-p)) > 5. This is true for every node independently. However, due to the fact that the degrees of nodes in a graph are not independent, we aim in this paper to test whether the degrees of per node collectively in the Erdős-Rényi graph have a multivariate normal distribution MVN. A chi square goodness of fit test for the hypothesis that binomial is a distribution for the whole set of nodes is rejected because of the dependence between degrees. Before testing MVN we show that the covariance and correlation between the degrees of any pair of nodes in the graph are p(1-p) and 1/(n-1), respectively. We test MVN considering two assumptions: independent and dependent degrees, and we obtain our results based on the percentages of rejected statistics of chi square, the p-values of Anderson Darling test, and a CDF comparison. We always achieve a good fit of multivariate normal distribution with large values of n and p, and very poor fit when n or p are very small. The approximation seems valid when np ≥ 10. We also compare the maximum likelihood estimate of p in MVN distribution where we assume independence and dependence. The estimators are assessed using bias, variance and mean square error.
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