International Workshop on Combinatorial Algorithms

Combinatorial Algorithms pp 185-196 | Cite as

Contagious Sets in Dense Graphs

  • Daniel Freund
  • Matthias Poloczek
  • Daniel Reichman
Conference paper
Part of the Lecture Notes in Computer Science book series (LNCS, volume 9538)


We study the activation process in undirected graphs known as bootstrap percolation: a vertex is active either if it belongs to a set of initially activated vertices or if at some point it had at least r active neighbors, for a threshold r that is identical for all vertices. A contagious set is a vertex set whose activation results with the entire graph being active. Let m(Gr) be the size of a smallest contagious set in a graph G. We examine density conditions that ensure that a given n-vertex graph \(G=(V,E)\) has a small contagious set. With respect to the minimum degree, we prove that if G has minimum degree \(n{\slash }2\) then \(m(G,2)=2\). We also provide tight upper bounds on the number of rounds until all nodes are active.

For \(n \ge k \ge r\), we denote by M(nkr) the maximum number of edges in an n-vertex graph G satisfying \(m(G,r)>k\). We determine the precise value of M(nk, 2) and M(nkk) assuming that n is sufficiently large compared to k.


Bootstrap percolation Target set selection Extremal graph theory 


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Copyright information

© Springer International Publishing Switzerland 2016

Authors and Affiliations

  • Daniel Freund
    • 1
  • Matthias Poloczek
    • 2
  • Daniel Reichman
    • 3
  1. 1.Center for Applied MathematicsCornell UniversityIthacaUSA
  2. 2.School of Operations Research and Information EngineeringCornell UniversityIthacaUSA
  3. 3.Department of Computer ScienceCornell UniversityIthacaUSA

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