The framework of Temporal constraint Satisfaction Problems (TCSP) has been proposed for representing and processing temporal knowledge. Deciding consistency of TC-SPs is known to be intractable. As demonstrates in this paper, even local consistency algorithms like path-consistency can be exponential due to the fragmentation problem. We present t wo new polynomial approximation algorithms, Upper-Lower-Tightening (ULT) and Loose-Path-Consistency (LPC), which are ecient y et eective in detecting inconsistencies and reducing fragmentation. The experiments we performed on hard problems in the transition region show that LPC is the superior algorithm. When incorporated within backtrack search LPC is capable of improving performance by orders of magnitude. Problems involving temporal constraints arise in various areas such as temporal databases [6], diagnosis [11], scheduling [22, 2 1 ], planning [16], common-sense reasoning [25] and natural language understanding [1]. Several formalisms for expressing and reasoning about temporal constraints have been proposed; interval algebra [2], point algebra [29], Temporal Constraint Satisfaction Problems (TCSP) [7] and the model of combined quantitative and qualitative constraints [17, 12].The two t ypes of Temporal Constraint Networks that have emerged are qualitative [2] and quantitative [7]. In the qualitative model, variables are time intervals and the constraints are qualitative. In the quantitative model, variables represent time points and the constraints are metric. Subsequently, these two t ypes were combined into a single model [17,12]. In this paper we build upon the model proposed by [17], whose variables are either points or intervals and involves three types of constraints: metric point-point and qualitative pointinterval and interval-interval.Answering queries in constraint processing reduces to the tasks of determining consistency, computing a consistent scenario and computing the minimal network. When time is represented by rational numbers 1 , deciding consistency is in NP-complete [7,17]. For qualitative networks, computing the minimal network is in NP-hard [10,7]. In both qualitative and quantitative models, the source of complexity stems from allowing disjunctive relationships between pairs of variables. Such constraints often arise in many applications, as demonstrated by the following example:
Interactive television (iTV) is an evolutionary merging of digital TV and the internet. iTV technology offers new powerful ways for consumers to interact with content and service providers. In Europe, iTV has gained significant traction during the turn of the century. For example, about 500,000 viewers signed up for SkyDigital's email service during 2000. In another example, Nickelodeon's "Watch Your Own Week" voting application was available to SkyDigital viewers during Oct 22-27 2001. While only 100,000 votes were anticipated for the whole week, this goal was reached within two days; a total of 578,000 votes were recorded for the week. Today, Europe counts tens of millions of iTV consumers.The iTV Handbook is a broad overview of the business and technical issues, and could be used as a textbook for an introductory technical course: it lays out the current thinking on commercially viable uses of iTV, surveys the related technical standards, and describes a broad range of technologies and the relationships among them. A whole chapter is devoted to the big picture of the iTV food chain, and another key chapter is devoted to a survey of media streaming methods. A "file system in the sky" is described which is the broadcasting equivalent of the network file system, and can be used to eliminate the notorious "hot spot" encountered when millions of receivers are trying to access a small set of pages. The book provides many rare insights into the nuts and bolts of the technologies being used. For example, the book presents part of the theoretical foundation for MP3 compression, and describes in detail many popular file formats used to deliver iTV content, including GIF, QuickTime, AVI, and ZIP.
Time is fundamental in representing and reasoning about changing domains. A proper temporal representation requires characterizing two notions: (I) time itself, and (2) tempoml incidence, i.e. the domainindependent properties for the truth-value of fiuents and events through out time. FormaUy defining them involves some problematic issues such as (i) the expression of instantaneous events and instantaneous holding of fluents, (ii) the dividing instant problem and (iii) the formalization of the properties for non-instantaneous holding affluents.This paper discusses how previous attempts fail to address all these issues and presents a simple theory of time and temporal incidence which satisfactorily overcomes all of them.Our theory of time, called JV, is based on having instants and periods at equal level. Our theory of temporal incidence is defined upon XV. Its key insight is the distinction between continuous and discrete fluents.' Hybrid systems are interesting since manydailyusedelectro-mechanical devices are euit-
In this work we combine logic programming and temporal constraint processing techniques. We propose TCLP, which augments logic programs with temporal constraints. Known algorithms for processing disjunctions in Temporal Constraint Networks are applied. We identify a decidable fragment called, Simple TCLP, which can be viewed as extending Datalog with limited functions to accommodate intervals of occurrence and temporal constraints between them. Some of the restrictions introduced by Simple TCLP are overcome by a syntactic structure which provides with the benefits of reihcation. The latter allows quantification on tem poral occurrences and relation symbols.
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