DIMACS10 set: clustering/preferentialAttachment                    
source: http://www.cc.gatech.edu/dimacs10/archive/clustering.shtml 
                                                                   
This graph has been generated following a preferential attachment  
process (see Barabási and Albert, "Emergence of scaling in random  
networks", Science, 1999). Starting with a clique of five vertices,
the vertices are successively added to the graph. Each new vertex  
chooses exactly five neighbors among the existing vertices, such   
that the probability of choosing a particular vertex is            
proportional to its degree. In our implementation, a vertex can    
choose a neighbour only once, such that the resulting random graph 
is guaranteed to be simple.