A characterization of answer sets for logic programs

Checking if a program has an answer set, and if so, compute its answer sets are just some of the important problems In answer set logic progremming. Solving these problems using Gelfond and Llfschltz＇s original definition of answer sets Is not an easy task. Alternative charaoterlzatlons of answer sets for nested logic programs by Erdem and Llfschltz, Lee and Llfschltz, and You at el. are based on the completion semantics and various notions of tlghtnese. However, the notion of tightness Is a local notion In the sense that for different answer sets there are, In general, different level mappings capturing their tlghtnese. This makes It hard to be used In the deelgn of algorithms for computing answer sets. This paper proposes a charecterization of answer sets based on sets of generetlng rules. From this charaoterlzation new algorithms are derived for computing answer sets and for performing some other reasoning teaks. As an application of the charecterlzatlon a sufficient and necessary condition for the equivalence between answer set sementics and completion semantics has been proven, and a basic theorem Is shown on computing answer sets for nested logic programs baaed on an extended notion of loop formulas. These results on tlghtnese and loop formulas are more general than that in You and Lin＇a work.

A Logic-Program-Based Negotiation Mechanism

This paper presents a logic-program-based mechanism of negotiation between two agents. In this mechanism an extended logic program （ELP） is regarded as an agent. The negotiation process between two agents is then modelled as multiple encounters between two ELPs, each of which selects an answer set as its initial demand. Both agents mutually revise the original sets of demands through accepting part of the opponent＇s demand and/or giving up part of its own demand. The overall dynamics can be regarded as mutual updates between two extended logic programs. A deal to achieve an appropriate negotiation solution is put forward. The conditions of existence and terminability of an appropriate negotiation are given. Properties of a negotiation solution are discussed, including its weak Pareto optimality.

Logic Programs,Compatibility and Forward Chaining Construction

Logic programming under the stable model semantics is proposed as a non-monotonic language for knowledge representation and reasoning in artificial intelligence. In this paper, we explore and extend the notion of compatibility and the A operator, which were first proposed by Zhang to characterize default theories. First, we present a new characterization of stable models of a logic program and show that an extended notion of compatibility can characterize stable submodels. We further propose the notion of weak auto-compatibility which characterizes the Normal Forward Chaining Construction proposed by Marek, Nerode and Remmel. Previously, this construction was only known to construct the stable models of FC-normal logic programs, which turn out to be a proper subclass of weakly auto-compatible logic programs. We investigate the properties and complexity issues for weakly auto-compatible logic programs and compare them with some subclasses of logic programs.