Norman Y. Foo

Infinitary belief revision

This paper extends the AGM theory of belief revision to accommodate infinitary belief change. We generalize both axiomatization and modeling of the AGM theory. We show that most properties of the AGM belief change operations are preserved by the generalized operations whereas the infinitary belief change operations have their special properties. We prove that the extended axiomatic system for the generalized belief change operators with a Limit Postulate properly specifies infinite belief change. This framework provides a basis for first-order belief revision and the theory of revising a belief state by a belief state.

A denotational semantics for Prolog

A denotational semantics is presented for the language Pro.og. Metapredicates are not considered. Conventional control sequencing is assumed for Prologs execution. The semantics is nonstandard, and goal continuations are used to explicate the sequencing.

Collusion Detection in Online Rating Systems

Online rating systems are subject to unfair evaluations. Users may try to individually or collaboratively promote or demote a product. Collaborative unfair rating, i.e., collusion, is more damaging than individual unfair rating. Detecting massive collusive attacks as well as honest looking intelligent attacks is still a real challenge for collusion detection systems. In this paper, we study impact of collusion in online rating systems and asses their susceptibility to collusion attacks. The proposed model uses frequent itemset mining technique to detect candidate collusion groups and sub-groups. Then, several indicators are used for identifying collusion groups and to estimate how damaging such colluding groups might be. The model has been implemented and we present results of experimental evaluation of our methodology.

Reasoning about action: an argumentation-theoretic approach

We present a uniform non-monotonic solution to the problems of reasoning about action on the basis of an argumentation-theoretic approach. Our theory is provably correct relative to a sensible minimisation policy introduced on top of a temporal propositional logic. Sophisticated problem domains can be formalised in our framework. As much attention of researchers in the field has been paid to the traditional and basic problems in reasoning about actions such as the frame, the qualification and the ramification problems, approaches to these problems within our formalisation lie at heart of the expositions presented in this paper.

LPOD answer sets and nash equilibria

Logic programs with ordered disjunctions (LPODs) are natural vehicles for expressing choices that have a preference ordering. They are extensions of the familiar extended logic programs that have answer sets as semantics. In game theory, players usually prefer strategies that yield higher payoffs. Since strategies are choices, LPODs would seem to be a suitable logical formalism for expressing some game-theoretic properties. This paper shows how pure strategy normal form games can be encoded as LPODs in such a way that the answer sets that are mutually most preferred by all players are exactly the Nash equilibria. A similar result has been obtained by researchers using a different, but related, logical formalism, viz., ordered choice logic programs that were used to encode extensive games.

Measuring similarity in belief revision

ThePossible Models Approach (PMA) introduced by Winslett, proposes, among other things, a domain-independent criterion for measuringsimilarity between different states of a dynamic system. The concept of similarity between states (possible worlds) appears also in the area of Belief Revision in the form of asystem of spheres. Systems of spheres are in turn connected to epistemic entrenchments by means of the AGM revision functions that the two structures induce. In view of these connections, in this article we study the implications of adopting PMA’s criterion of similarity in the context of Belief Revision. More precisely, we formulate conditions that capture PMA’s criterion of similarity in terms of systems of spheres ( PMA systems of spheres) and we provide an axiomatic characterization of the class of epistemic entrenchments corresponding to systems of spheres that comply with PMA’s criterion of similarity (PMA epistemic entrenchments). We also discuss some interesting properties of the class of PMA system of spheres and PMA epistemic entrenchments. Our study is primarily motivated by the role that PMA epistemic entrenchments can play in Reasoning about Action.

EMERGENCE AND COMPUTATION

By introducing a computational component into system complexity theory it is argued that the conflict between holistic and reductionist views of systems can be partially reconciled. The prospects and limitations of this reconciliation are discussed.

Updates with Disjunctive Information: From Syntactical and Semantical Perspectives

The possible models approach is a classical minimal change semantics for knowledge base update, which provides an exclusive interpretation for disjunctive information in updates. It has been recognized that the exclusive interpretation for disjunction may be problematic under some circumstances. In this paper, we investigate inclusive interpretations for disjunctions in updates from both syntactical and semantical viewpoints. In particular, we propose two approaches for disjunctive update—the minimal change with exceptions (MCE) and the minimal change with maximal disjunctive inclusions (MCD). Both approaches provide inclusive interpretations for disjunctions in updates, though the first is syntax‐based and the second is model‐theoretic. We then characterize the MCE and MCD in terms of alternative minimal change criteria and relate them to traditional Katsuno and Mendelzons update postulates.

Deriving Invariants and Constraints from Action Theories

Recent work on reasoning about action has shown that there exists an interesting connection between action specifications and state constraints — it is possible to extract state constraints from action specifications. This work provides us another way to describe the behaviour of dynamic systems. In this paper, we address the problem of generating action invariants from action specifications, and generalizing action invariants into state constraints. We first propose a persistence-based formalism of actions, and show that the generation of action invariants is achieved from action specifications by reasoning about persistence. We then investigate the generalization of action invariants into state constraints. Blocks world examples illustrate the general procedure throughout.

Representation and querying of unfair evaluations in social rating systems

Social rating systems are subject to unfair evaluations. Users may try to individually or collaboratively promote or demote a product. Detecting unfair evaluations, mainly massive collusive attacks as well as honest looking intelligent attacks, is still a real challenge for collusion detection systems. In this paper, we study the impact of unfair evaluations in online rating systems. First, we study the individual unfair evaluations and their impact on the reputation of people calculated by social rating systems. We then propose a method for detecting collaborative unfair evaluations, also known as collusion. The proposed model uses frequent itemset mining technique to detect the candidate collusion groups and sub-groups. We use several indicators to identify collusion groups and to estimate how destructive such colluding groups can be. The approaches presented in this paper have been implemented in prototype tools, and experimentally validated on synthetic and real-world datasets.