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These are my most significant published works. They are classified in:


Lluís Vila. On Temporal Representation and Reasoning in Knowledge-based Systems, Series Monografies de l'IIIA, 3, 1995. You can get the abstract by clicking here and purchase the book by contacting Eva López at

Journal papers

Eddie Schwalb, Lluís Vila. Temporal Constraints: A Survey, Constraints: An International Journal (ISBN 1383-7133), vol. 3, n. 2/3, pp. 129-149, June 1998.

Abstract: Temporal Constraint Satisfaction is an information technology useful for representing and answering queries about the times of events and the temporal relations between them. Information is represented as a Constraint Satisfaction Problem (CSP) where variables denote event times and constraints represent the possible temporal relations between them. The main tasks are two: (i) deciding consistency, and (ii) answering queries about scenarios that satisfy all constraints. This paper overviews results on several classes of Temporal CSPs: qualitative interval, qualitative point, metric point, and some of their combinations. Research has progressed along three lines: (i) identifying tractable subclasses, (ii) developing exact search algorithms, and (iii) developing polynomial-time approximation algorithms. Most available techniques are based on two principles: (i) enforcing local consistency (e.g. path-consistency), and (ii) enhancing naive backtracking search.

Lluís Vila, Hajime Yoshino. Time in Automated Legal Reasoning, Information and Communications Technology Law (ISBN 1360-0834), vol. 7, n 3, pp. 173-197, 1998.

Abstract: Despite the ubiquity of time and temporal references in legal texts, their formalization has often been disregarded or addressed in an ad hoc manner. We address this issue from the standpoint of the research done in temporal representation and reasoning in AI. We identify the temporal requirements of legal domains and propose a temporal representation framework for legal reasoning independent of (i) the underlying representation language and (ii) the specific legal reasoning application. The approach is currently being used in a rule-based language for an application in commercial law.

Lluís Vila, Han Reichgelt. The Token Reificacion Approach to Temporal Reasoning, Artificial Intelligence Vol. 83 (1), May 1996.

Abstract: The approach to temporal reasoning which has proven most popular in AI is the reified approach. In this approach, one introduces names for events and states and uses special predicates to assert that an event or state occurs or holds at a particular time. However, recently the reified approach has come under attack, both on technical and on ontological grounds. Thus, it has been claimed that at least some reified temporal logics do not give one more expressive power than provided by alternative approaches. Moreover, it has been argued that the reification of event and state types in reified temporal logics, rather than event and state tokens, makes the ontology more complicated than necessary.

In this paper, we present a new reified temporal logic, called TRL, which we believe avoids most of these objections. It is based on the idea of reifying event tokens instead of event types. However, unlike other such attempts, our logic contains ``meaningful'' names for event tokens, thus allowing us to quantify over all event tokens that meet a certain criterion. The resulting logic is more expressive than alternative approaches. Moreover, it avoids the ontologically objectionable reification of event types, while staying within classical predicate logic.

Lluís Vila, Lluís Godo. On Fuzzy Temporal Constraint Networks, Mathware and Soft Computing Vol. 1, n. 3, pp. 315-334, 1994.

Abstract: Temporal Constraint Networks are a well-defined, natural and efficient formalism for representing temporal knowledge based on metric temporal constraints. They support the representation of both metric and some qualitative temporal relations and are provided with efficient algorithms based on CSP techniques. Recently, a generalization based on fuzzy sets has been proposed in order to cope with vagueness in temporal relations. In this paper we generalize some earlier definitions for Fuzzy Temporal Constraint Networks, we identify and define ``interesting'' queries in a fuzzy temporal constraint network, and explore a method for efficiently computing them in a specific case. Further analysis of some measures on possibility distributions turns out to be fundamental in order to precisely determine some of these queries. We discuss the advantages and shortcomings of various choices and propose specific alternatives which satisfactorily avoid the problems of previous proposals. The results presented in this paper can be useful in the design of a system for temporal reasoning under uncertainty. For instance, we have applied them to define a possibilistic temporal logic in Possibilistic Temporal Reasoning based on Fuzzy Temporal Constraints where approximate and temporal representation and reasoning are consistently combined.

Lluís Vila. A Survey on Temporal Reasoning in Artificial Intelligence, AI Communications, Vol. 7, pp. 4-28, March 1994 (Note: This is not an up-to-date survey).

Abstract: The notion of time is ubiquitous in any activity that requires intelligence. In particular, several important notions like change, causality, action are described in terms of time. Therefore, the representation of time and reasoning about time is of crucial importance for many Artificial Intelligence systems. Specifically during the last 10 years, it has been attracting the attention of many AI researchers. In this survey, the results of this work are analysed. Firstly, Temporal Reasoning is defined. Then, the most important representational issues which determine a Temporal Reasoning approach are introduced: the logical form on which the approach is based, the ontology (the units taken as primitives, the temporal relations, the algorithms that have been developed, ...) and the concepts related with reasoning about action (the representation of change, causality, action, ...). For each issue the different choices in the literature are discussed.

Book Contributions

Lluís Vila, Francisco Toledo, Salvador Moreno. Modelling Time in Qualitative Reasoning about Physical Systems. In Current Trends in Qualitative Reasoning and Applications, chapter 4, Ed: N. Piera, 1995.

Lluís Vila, Carles Sierra, Antonio Martínez and Joan Climent. Intelligent Process Control by means of Expert Systems and Machine Vision. In Industrial and Engineering Application of Artificial Intelligence and Expert Systems. Lecture notes in Artificial Intelligence, Ed: F. Belli and F.J. Radermacher, Springer Verlag, 1992.

Conference Papers

Daniel Frost, Irina Rish and Lluís Vila. Summarizing CSP hardness with continuous probability distributions AAAI'97.

Abstract: We present empirical evidence that the effort required to solve CSPs randomly generated at the 50% satisfiable point and solved by backtracking based algorithms, can be approximated by two standard continuous probability distributions functions. Solvable problems are quite well modelled by the Weibull distribution function, and unsolvable problems by the inverse Gaussian distribution.

Lluís Godo, Lluís Vila Possibilistic Temporal Reasoning based on Fuzzy Temporal Constraints, IJCAI'95.

Abstract: In this paper we propose a propositional temporal language based on fuzzy temporal constraints which turns out to be expressive enough for domains -like many coming from medicine- where knowledge is of propositional nature and an explicit handling of time, imprecision and uncertainty are required. The language is provided with a natural possibilistic semantics to account for the uncertainty issued by the fuzziness of temporal constraints. We also present an inference system based on specific rules dealing with the temporal constraints and a general fuzzy modus ponens rule whereby behaviour is shown to be sound. The analysis of the different choices as fuzzy operators leads us to identify the well-known Lukasiewicz implication as very appropriate to define the notion of possibilistic entailment, an essential element of our inference system.

Lluís Vila, Lluís Godo. Query-Answering in Fuzzy Temporal Constraint Networks, FUZZIEEE/IFES'95.

Lluís Vila, Gonzalo Escalada-Imaz. Temporal Token Calculus: a Temporal Reasoning Approach for Knowledge-Based Systems, Joint Conference on Declarative Programming (GULP-PRODE'94).

Abstract: Most of AI research on temporal reasoning has been devoted to either exploring constraint-based temporal deduction techniques or investigating diverse logics extended with time. Nevertheless the formal study of deductive systems for such temporal extended logics has received little attention. This paper presents a general framework for temporal reasoning in knowledge-based systems resulting from embedding a temporal reasoner into a general calculus. From a representational point of view it is based on the notions of temporal token and temporal constraint. The logic is formally defined as a particular many-sorted predicate calculus and provided with an complete and sound inference system composed of non-temporal and temporal inference rules. Moreover, a deductive procedure is presented and analyzed. It is a general forward chaining algorithm for which soundness and completeness are guaranteed.

Lluís Vila, Carles Sierra. IPCES: Intelligent Process Control by means of Expert Systems. International Conference on Artificial Intelligence, Expert Systems and Natural Language (AVIGNON'92), 1992.

Relevant International Workshop Papers

Lluís Vila, Eddie Schwalb. A Theory of Time and Temporal Incidence based on Instants and Periods, TIME'96.

Abstract: Time is fundamental in representing and reasoning about changing domains. A proper temporal representation requires characterizing two notions: (1) time> itself, and (2) temporal incidence, i.e. the domain-independent properties for the truth-value of fluents and events throughout time. There are some problematic issues such as the expression of instantaneous events and instantaneous holding of fluents, the specification of the properties for the temporal holding of fluents and the Dividing Instant Problem.

This paper presents a theory of time and temporal incidence which is more natural than its predecessors and satisfactorily addresses the issues above. Our theory of time, called IP, is based on having instants and periods at equal level. We define a theory of temporal incidence upon it whose main original feature is the distinction between continuous and discrete fluents.

Eddie Schwalb, Lluís Vila. Logic Programming with Temporal Constraints, TIME'96.

Lluís Vila, Hajime Yoshino. Temporal Representation for Legal Reasoning, 3rd Intl. Workshop on a Legal Expert System for the CISG, Washington D.C., 1995.

Lluís Vila. IP: A Theory of Time based on Instants and Periods, ECAI'94 Workshop on Spatio-temporal Reasoning.

Abstract: Instants have been criticised as temporal primitive for common-sense reasoning on ontological -for being just abstract entities not concerned with common-sense- and semantical - the Divided Instant Problem (DIP)- arguments. Therefore, period-based theories have received special attention. In this paper we provide arguments for incorporating instants to the time ontology. We present an axiomatization of time based on instants and periods (IP), characterize all the models of the theory, and explore its relations with other Period- and Instant-Period-based theories appeared in the AI community. Finally, we discuss its suitability for supporting temporal knowledge representation and the Divided Instant Problem is revisited.

Lluís Vila. Instants, Periods and the Dividing Instant Problem. IMACS International Workshop on Qualitative Reasoning and Decision Technologies (QUARDET'93), 1993.

Relevant Technical Reports

Lluís Vila. An Analysis of the main Approaches to Temporal Reasoning in AI, IIIA technical report 94-14, 1994 (Note: This is not an up-to-date survey).

Research Interests | Research Activities | Papers | Teaching | Resume Sketch | Personal

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Last update: 1 June 99