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 programming. Solving these problems using Gelfond and Lifschitz's original definition of answer sets is not an easy task. Alternative characterizations of answer sets for nested logic pro- grams by Erdem and Lifschitz, Lee and Lifschitz, and You et al. are based on the completion semantics and various notions of tightness. 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 tightness. This makes it hard to be used in the design of algorithms for computing answer sets. This paper proposes a characterization of answer sets based on sets of generating rules. From this char- acterization new algorithms are derived for computing answer sets and for per- forming some other reasoning tasks. As an application of the characterization a sufficient and necessary condition for the equivalence between answer set seman- tics and completion semantics has been proven, and a basic theorem is shown on computing answer sets for nested logic programs based on an extended notion of loop formulas. These results on tightness and loop formulas are more general than that in You and Lin's work.
Based on logic programs,authorization conflicts and resolution strategies are analyzed through the explanation of some examples on the health care sector. A resolution scheme for han-dling conflicts in high level authorization specification by using logic program with ordered disjunction (LPOD) is proposed. The scheme is useful for solving conflicts resulted from combining positive and negative authorization,complexity of authorization management,and less clarity of the specification. It can well spec-ify kinds of conflicts (such as exceptional conflicts,potential con-flicts),and is based on literals and dependent contexts. Thus it is expressive and available. It is shown that authorizations based on rules LPOD is very important both in theory and practice.
