
From:  Carlos Eduardo 
Subject:  [igraph] Igraph commands about directed and undirected graphs 
Date:  Mon, 12 Sep 2011 09:49:33 0300 
Hi, Once lots of commands in igraph demand to choose whether network is directed or undirected, I need some help to understand the follow triad situations (attached a simple diagram illustrating): Situation 1 (all edges directed): A=>B B=>A B=>C C=>A Situation 2 (the two edges in situation 1, between A and B, were simplified to just one edge undirected): A<=>B (undirected) B=>C (directed) C=>A (directed) I did measures of density, centrality degree, indegree, outdegree, betweenness and closeness. The difference was noticed only in centrality degree (situation 1: A and B have 3 / situation 2: A and B have 2). So, in situation 2, I have valued the undirected edge to 2 and the others (directed) to 1. After that, using edge strength in centrality degree measure, the result was equal to both situations. a) Is the simplification illustrated in situation 2 correct (two directed edges between A and B simplified to one undirected edge)? b) Is there any problem about building networks as illustrated in situation 2 (directed and undirected edges handled/represented together)? c) Is the procedure about edge value correct (to obtain the same centrality degree in both situations)? Is it really necessary? d) Could I point in igraph commands situation 2 (with undirected and directed edges together) as a directed graph? Thanks indeed in advance, Cadu 
Directed and undirected graphs.png
Description: PNG image
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