Rank-Based Attachment Leads to Power Law Graphs

  • Authors:
  • Jeannette Janssen;PaweŁ PraŁat

  • Affiliations:
  • janssen@mathstat.dal.ca;pralat@math.wvu.edu

  • Venue:
  • SIAM Journal on Discrete Mathematics
  • Year:
  • 2010

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Abstract

We investigate the degree distribution resulting from graph generation models based on rank-based attachment. In rank-based attachment, all vertices are ranked according to a ranking scheme. The link probability of a given vertex is proportional to its rank raised to the power $-\alpha$, for some $\alpha\in(0,1)$. Through a rigorous analysis, we show that rank-based attachment models lead to graphs with a power law degree distribution with exponent $1+1/\alpha$ whenever vertices are ranked according to their degree, their age, or a randomly chosen fitness value. We also investigate the case where the ranking is based on the initial rank of each vertex; the rank of existing vertices changes only to accommodate the new vertex. Here, we obtain a sharp threshold for power law behavior. Only if initial ranks are biased towards lower ranks, or chosen uniformly at random, do we obtain a power law degree distribution with exponent $1+1/\alpha$. This indicates that the power law degree distribution often observed in nature can be explained by a rank-based attachment scheme, based on a ranking scheme that can be derived from a number of different factors; the exponent of the power law can be seen as a measure of the strength of the attachment.