Ramón Béjar

Publications (51) View all

• Conference Proceeding: The interface between P and NP in signed CNF formulas

  C. Ansotegui, R. Bejar, A. Cabiscol, F. Manya
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  ABSTRACT: We first define a new class of signed CNF formulas and prove that its satisfiability problem is NP-complete. We then study in detail the interface between P and NP in two many-valued satisfiability problems: Mono+pPartiallySigned-2SAT and Regular+pSigned-2SAT. We show that such problems smoothly interpolate between P and NP by mixing together a polynomial and an NP-complete problem, and identify phase transition behavior in each of these problems.
  Multiple-Valued Logic, 2004. Proceedings. 34th International Symposium on; 06/2004
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  Available from: Ramón Béjar

  Article: Distributed Constraint Satisfaction in a Wireless Sensor Tracking System

  Ramon Bejar, Bhaskar Krishnamachari, Carla Gomes, Bart Selman
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  ABSTRACT: This paper describes our ongoing work on an interesting distributed constraint satisfaction problem (DCSP), SensorCSP, that is based on a system of wireless sensors tracking multiple mobile nodes. We present some preliminary results showing that the source of combinatorial complexity in this problem is closely linked to the level of communication in the system. This DCSP lends itself naturally to two models - one in which agents are associated with the sensors, and one in which agents are associated with the mobile nodes. We show that these models are duals of each other, and discuss how they di#er in the number of intra and inter-agent constraints and how this might a#ect the cost of finding a distributed solution. We also suggest that a careful distinction must be made between explicit and implicit inter-agent constraints in this problem domain as this might a#ect the communication costs and the scalability of a distributed solution. 1
  08/2001;
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  Article: Distributed Constraint Satisfaction and the Bounds on Resource Allocation in Wireless Networks

  Bhaskar Krishnamachari, Ramon Bejar, Stephen Wicker
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  ABSTRACT: In this paper we consider medium access scheduling in ad hoc networks as a distributed constraint satisfaction problem (DCSP), and present experimental results on the solvability and complexity of this problem. We show that there are "phase transitions" in solvability and complexity with respect to the transmission power of the wireless nodes. The phase transition curves indicate that there is a critical maximum power level for certain arrangements of nodes and a given availability of spectrum in an ad hoc network beyond which the problem of channel allocation becomes intractable. 1
  07/2001;
• Article: Automated monitoring of medical protocols: a secure and distributed architecture.

  T Alsinet, C Ansótegui, R Béjar, C Fernández, F Manyà
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  ABSTRACT: The control of the right application of medical protocols is a key issue in hospital environments. For the automated monitoring of medical protocols, we need a domain-independent language for their representation and a fully, or semi, autonomous system that understands the protocols and supervises their application. In this paper we describe a specification language and a multi-agent system architecture for monitoring medical protocols. We model medical services in hospital environments as specialized domain agents and interpret a medical protocol as a negotiation process between agents. A medical service can be involved in multiple medical protocols, and so specialized domain agents are independent of negotiation processes and autonomous system agents perform monitoring tasks. We present the detailed architecture of the system agents and of an important domain agent, the database broker agent, that is responsible of obtaining relevant information about the clinical history of patients. We also describe how we tackle the problems of privacy, integrity and authentication during the process of exchanging information between agents.
  Artificial Intelligence in Medicine 04/2003; 27(3):367-92. · 1.35 Impact Factor
• Chapter: Minimal and Redundant SAT Encodings for the All-Interval-Series Problem

  T. Alsinet, R. Béjar, A. Cabiscol, C. Fernàndez, F. Manyà
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  ABSTRACT: The SAT encodings defined so far for the all-interval-series (ais) problem are very hard for local search but rather easy for systematic algorithms. We define different SAT encodings for the ais problem and provide experimental evidence that this problem can be efficiently solved with local search methods if one chooses a suitable SAT encoding.
  12/2001: pages 139-144;