added reading material for week 2

reading_material/week_2/README.md

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+# Reading material for Week 2 (26/02/2018)
+
+The lecture in Week 2 will be focused on the computation of the 
+connected components of a graph, of pairwise node distances, and 
+of various node centrality scores. 
+
+Please make sure to have read the references indicated below:
+
+
+  1. _Degree and Degree correlations_ 
+
+     * S. Boccaletti et al. "Complex Networks: Structure and
+       dynamics", Phys. Rep. 424 (2006) 175-308 **Only section 2.1.1
+       "Node degree, degree distributions and correlations"**
+       [paper here](./Boccaletti2006_physrep.pdf)
+  
+  2. _Definition of connected components associated to nodes and graphs_
+
+     * V. Nicosia et al. "Connected components in time-varying 
+       graphs", Chaos, vol 22, 2, 023101 doi:10.1063/1.3697996 
+       (2012) **Only section "II Components in Static graphs"**
+       [paper here](./Nicosia2012.pdf) 
+
+  3. _Paths, shortest paths, node distances, and betweenness_
+
+     * S. Boccaletti et al. "Complex Networks: Structure and
+       dynamics", Phys. Rep. 424 (2006) 175-308 **Only section 2.1.2
+       "Shortest path lengths, diameter and betweenness"**
+ 	[paper here](./Boccaletti2006_physrep.pdf)
+
+  4. _Eigenvector centrality and PageRank_
+
+     * P. Bonacich, "Power and Centrality: A Family of Measures", 
+       Am. J. Sociol. 92(5), (1987), 1170-1192, **Pages 1172-1176**
+       [paper here](./Bonacich1987-Centrality.pdf)
+   
+     * The PageRank Algorithm 
+       [Wikipedia page](https://en.wikipedia.org/wiki/PageRank#Algorithm)
+
+