10845241
On Odd and Even Cycles in Normal Logic Programs
An odd cycle of a logic program is a simple cycle that has an odd number of negative edges in the dependency graph of the program. Similarly, an even cycle is one that has an even number of negative edges. For a normal logic program that has no odd cycles, while it is known that such a program always has a stable model, and such a stable model can be computed in polynomial time, we show in this paper that checking whether an atom is in a stable model is NP-complete, and checking whether an atom is in all stable models is co-NP complete, both are the same as in the general case for normal logic programs. Furthermore, we show that if a normal logic program has exactly one odd cycle, then checking whether it has a stable model is NP-complete, again the same as in the general case. For normal logic programs with a fixed number of even cycles, we show that there is a polynomial time algorithm for computing all stable models. Furthermore, this polynomial time algorithm can be improved significantly if the number of odd cycles is also fixed.
30011892
Complexity of argument-based default reasoning with specificity
In this paper we investigate the complexity of Dung and Son's approach to default reasoning with specificity. We show that the three decision problems: existence of extensions, brave reasoning and cautious reasoning are generally complete for the second level of the polynomial hierarchy. For some restricted cases, e.g., Horn and 2CNF default theories, all the three problems are intractable although classical brave reasoning in these cases is tractable. Finally, we show that stratification can not reduce the complexity of brave and cautious reasoning although it guarantees the existence of extensions.
7900372
Complexity of the Unique Extension Problem in Default Logic
In this paper we analyze the problem of checking whether a default theory has a single extension. This problem is important for at least three reasons. First, if a theory has a single extension, nonmonotonic inference can be reduced to entailment in propositional logic (which is computationally easier) using the set of consequences of the generating defaults. Second, a theory with many extensions is typically weak i.e., it has few consequences; this indicates that the theory is of little use, and that new information has to be added to it, either as new formulae, or as preferences over defaults. Third, some applications require as few extensions as possible (e.g. diagnosis). We study the complexity of checking whether a default theory has a single extension. We consider the combination of several restrictions of default logics: seminormal, normal, disjunction-free, unary, ordered. Complexity varies from the first to the third level of the polynomial hierarchy. The problem of checking whether a theory has a given number of extensions is also discussed.
1096538
Complexity Results for 2CNF Default Theories
This paper is concerned with the complexity of default reasoning with 2CNF default theories (D,W) in which W is a set of literals and D is a set of 2CNF defaults. It is proved that the problem whether a literal occurs in at least one extension can be solved in polynomial time. But the problem whether a literal appears in all extensions is proved to be co-NP-complete. The complexity of some subclasses of 2CNF default theories is also analyzed.
33218482
Some Algorithms for Extension Computation of Nonmonotonic Rule Systems
In this paper, we will place our emphasis on the problem of existence of extensions and on finding more efficient algorithms for computing extensions of nonmonotonic rule systems. First, in order to dedicate further research into nonmonotonic rule systems a new characterization and some new results are given. This characterization and these results lead to new algorithms for computing extensions. Second, a class of nonmonotonic rule systems, called regular nonmonotonic rule systems, is developed. The existence of extensions and semi-monotonicity of regular nonmonotonic rule systems are proved. Third, we prove that each regular nonmonotonic rule system has at most 1.47^n extensions, where n is the number of elements occurring in the system.
8163164
Regular disjunction-free default theories
In this paper, the class of regular disjunction-free default theories is introduced and investigated. A transformation from regular default theories to normal default theories is established. The initial theory and the transformed theory have the same extensions when restricted to old variables. Hence, regular default theories enjoy some similar properties (e.g., existence of extensions, semi-monotonicity) as normal default theories. Then, a new algorithm for credulous reasoning of regular theories is developed. This algorithm runs in a time not more thanO(1. 45n), wheren is the number of defaults. In case of regular prerequisite-free or semi-2CNF default theories, the credulous reasoning can be solved in polynomial time. However, credulous reasoning for semi-Horn default theories is shown to be NP-complete although it is tractable for Horn default theories. Moreover, skeptical reasoning for regular unary default theories is co-NP-complete.
1211132
A Multi-Agent System for E-Business Processes Monitoring in a Web-Based Environment
In this paper, we present a multi-agent system MAGS for the e-business processes monitoring in a web-based environment. We classify the types of agents in MAGS by their monitoring capabilities. An algorithm is given to explain the mechanism of supervising and controlling the execution of business processes. An abstract model of alerts, which can give warnings of infringement on business policies, is proposed. Access control can also be realized by MAGS, which manifests in delivering different view of the business process to different roles participate in it. Being successfully adopted in a customer service management system, MAGS has been proven flexible and practical.
18186547
Experiences with a Mobile Testbed
This paper presents results from an eight-day network packet-trace of MosquitoNet. MosquitoNet allows users of laptop computers to switch seamlessly between a metropolitan-area wireless network and a wired network (10 Mbit/s Ethernet) available in offices and on-campus residences. Results include the amount of user mobility between the wired and wireless networks, the amount of mobility within the wireless network, an examination of application end-to-end delays, and an examination of overall packet loss and reordering in the wireless network. We find that the average mobile host switches between the wired and wireless networks 14 times during the trace and moves within the wireless network five times. Round trip latencies in the wireless network are very high, with a minimum of 0.2 seconds. Even higher end-to-end delays, of up to hundreds of seconds, are due to packet loss and reordering. These delays cause users to change their usage patterns when connected to the wireless network. We conclude that latency is a critical problem in the wireless network.
2194617
Model Checking Dynamic UML Consistency
UML is widely accepted and extensively used in software modeling. However, using different diagrams to model different aspects of a system brings the risk of inconsistency among diagrams. In this paper, we investigate an approach to check the consistency between the sequence diagrams and statechart diagrams using the SPIN model checker. To deal with the hierarchy structure of statechart diagrams, we propose a formalism called Split Automata, a variant of automata, which is helpful to bridge the statechart diagrams to SPIN efficiently. Compared with the existing work on model checking UML which do not have formal verification for their translation from UML to the model checker, we formally define the semantics and prove that the automatically translated model (i.e. Split Automata) does simulate the UML model. In this way, we can guarantee that the translated model does represent the original model.
15922787
Supporting Mobility in MosquitoNet
The goal of the MosquitoNet project is to provide continuous Internet connectivity to mobile hosts. Mobile hosts must be able to take advantage of the best network connectivity available in any location, whether wired or wireless. We have implemented a mobile IP system that supports seamless switching between different networks and communication devices. In contrast to previous approaches to mobile IP, we believe mobile hosts should not assume any explicit mobility support from the networks they visit, aside from basic Internet connectivity. This decision places extra responsibilities on the mobile hosts themselves. In this paper, we describe the design and implications of such a system. Measurements of our implementation show that the inherent overhead to switch networks (below 10ms) is insignificant compared to the time required to bring up a new communication device.
15041582
Towards the Semantics and Verification of BPEL4WS
In this paper, we discuss the semantics of BPEL4WS language which is a de facto standard for specifying and execution workflow specification for web service composition and orchestration. We propose a language @m-BPEL that includes most primitive and structured activities of BPEL4WS, and define its semantics. As the Timed Automata (TA) is powerful in designing real-time models with multiple clocks and has well developed automatic tool support, we define a map from @m-BPEL into composable TA. Therefore, the properties we want to check can be verified in TA network correspondingly. Furthermore, we prove that the mapping from @m-BPEL to TA is a simulation, which means that the TA network simulates correctly the corresponding @m-BPEL specification. The case study with model checker Uppaal shows that our method is effective, and a Java supporting tool based on Uppaal model checker engine has been developed.