The price of defense

  • Authors:
  • Marios Mavronicolas;Loizos Michael;Vicky Papadopoulou;Anna Philippou;Paul Spirakis

  • Affiliations:
  • Department of Computer Science, University of Cyprus, Nicosia, Cyprus;Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA;Department of Computer Science, University of Cyprus, Nicosia, Cyprus;Department of Computer Science, University of Cyprus, Nicosia, Cyprus;,Research Academic Computer Technology Institute (RACTI), Rion, Patras, Greece

  • Venue:
  • MFCS'06 Proceedings of the 31st international conference on Mathematical Foundations of Computer Science
  • Year:
  • 2006

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Abstract

We consider a strategic game with two classes of confronting randomized players on a graph G(V, E): νattackers, each choosing vertices and wishing to minimize the probability of being caught, and a defender, who chooses edges and gains the expected number of attackers it catches. The Price of Defense is the worst-case ratio, over all Nash equilibria, of the optimal gain of the defender over its gain at a Nash equilibrium. We provide a comprehensive collection of trade-offs between the Price of Defense and the computational efficiency of Nash equilibria. – Through reduction to a Two-Players, Constant-Sum Game, we prove that a Nash equilibrium can be computed in polynomial time. The reduction does not provide any apparent guarantees on the Price of Defense. – To obtain such, we analyze several structured Nash equilibria: – In a Matching Nash equilibrium, the support of the defender is an Edge Cover. We prove that they can be computed in polynomial time, and they incur a Price of Defense of α(G), the Independence Number of G. – In a Perfect Matching Nash equilibrium, the support of the defender is a Perfect Matching. We prove that they can be computed in polynomial time, and they incur a Price of Defense of $\frac{|V|}{2}$. – In a Defender Uniform Nash equilibrium, the defender chooses uniformly each edge in its support. We prove that they incur a Price of Defense falling between those for Matching and Perfect Matching Nash Equilibria; however, it is ${\cal NP}$-complete to decide their existence. – In an Attacker Symmetric and Uniform Nash equilibrium, all attackers have a common support on which each uses a uniform distribution. We prove that they can be computed in polynomial time and incur a Price of Defense of either $\frac{|V|}{2}$ or α(G).