Approximability of dense and sparse instances of minimum 2-connectivity, TSP and path problems
SODA '02 Proceedings of the thirteenth annual ACM-SIAM symposium on Discrete algorithms
Searching for a black hole in arbitrary networks: optimal mobile agent protocols
Proceedings of the twenty-first annual symposium on Principles of distributed computing
Mobile Search for a Black Hole in an Anonymous Ring
DISC '01 Proceedings of the 15th International Conference on Distributed Computing
Mobile Agent Security - Issues and Directions
IS&N '99 Proceedings of the 6th International Conference on Intelligence and Services in Networks: Paving the Way for an Open Service Market
Time Limited Blackbox Security: Protecting Mobile Agents From Malicious Hosts
Mobile Agents and Security
Protecting Mobile Agents Against Malicious Hosts
Mobile Agents and Security
A Framework to Protect Mobile Agents by Using Reference States
ICDCS '00 Proceedings of the The 20th International Conference on Distributed Computing Systems ( ICDCS 2000)
Complexity of Searching for a Black Hole
Fundamenta Informaticae
Searching for a black hole in tree networks
OPODIS'04 Proceedings of the 8th international conference on Principles of Distributed Systems
Hardness and approximation results for black hole search in arbitrary graphs
SIROCCO'05 Proceedings of the 12th international conference on Structural Information and Communication Complexity
Locating and Repairing Faults in a Network with Mobile Agents
SIROCCO '08 Proceedings of the 15th international colloquium on Structural Information and Communication Complexity
Ping Pong in Dangerous Graphs: Optimal Black Hole Search with Pure Tokens
DISC '08 Proceedings of the 22nd international symposium on Distributed Computing
Searching for black-hole faults in a network using multiple agents
OPODIS'06 Proceedings of the 10th international conference on Principles of Distributed Systems
Distributed security algorithms by mobile agents
ICDCN'06 Proceedings of the 8th international conference on Distributed Computing and Networking
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A black hole is a highly harmful stationary process residing in a node of a network and destroying all mobile agents visiting the node without leaving any trace. The Black Hole Search is the task of locating all black holes in a network, through the exploration of its nodes by a set of mobile agents. In this paper we consider the problem of designing the fastest Black Hole Search, given the map of the network, the starting node and, possibly, a subset of nodes of the network initially known to be safe. We study the version of this problem that assumes that there is at most one black hole in the network and there are two agents, which move in synchronized steps. We prove that this problem is not polynomial-time approximable within $\frac{389}{388}$ (unless P=NP). We give a 6-approximation algorithm, thus improving on the 9.3-approximation algorithm from [3]. We also prove APX-hardness for a restricted version of the problem, in which only the starting node is initially known to be safe.