sentences:  In a Directed graph, in-assortativity (formula_8) and out-assortativity (formula_9) measure the tendencies of nodes to connect with other nodes that have similar in and out degrees as themselves, respectively.Extending this further, four types of assortativity can be considered (see).Adopting the notation of that article, it is possible to define four metrics formula_8, formula_11, formula_12, and formula_9.