The k-core and branching processes

  • Authors:
  • Oliver Riordan

  • Affiliations:
  • Royal society research fellow, department of pure mathematics and mathematical statistics, university of cambridge, cambridge cb3 0wb, uk (e-mail: o.m.riordan@dpmms.cam.ac.uk)

  • Venue:
  • Combinatorics, Probability and Computing
  • Year:
  • 2008

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Abstract

The k-core of a graph G is the maximal subgraph ofG having minimum degree at least k. In 1996, Pittel,Spencer and Wormald found the threshold λc forthe emergence of a non-trivial k-core in the random graphG(n, λ/n), and the asymptotic size ofthe k-core above the threshold. We give a new proof of thisresult using a local coupling of the graph to a suitable branchingprocess. This proof extends to a general model of inhomogeneousrandom graphs with independence between the edges. As an example,we study the k-core in a certain power-law or 'scale-free'graph with a parameter c controlling the overall density ofedges. For each k ≥ 3, we find the threshold value ofc at which the k-core emerges, and the fraction ofvertices in the k-core when c is ε above thethreshold. In contrast to G(n, λ/n),this fraction tends to 0 as ε➝0.