316,951 research outputs found
Changing Observations in Epistemic Temporal Logic
We study dynamic changes of agents' observational power in logics of
knowledge and time. We consider CTL*K, the extension of CTL* with knowledge
operators, and enrich it with a new operator that models a change in an agent's
way of observing the system. We extend the classic semantics of knowledge for
perfect-recall agents to account for changes of observation, and we show that
this new operator strictly increases the expressivity of CTL*K. We reduce the
model-checking problem for our logic to that for CTL*K, which is known to be
decidable. This provides a solution to the model-checking problem for our
logic, but its complexity is not optimal. Indeed we provide a direct decision
procedure with better complexity
Non-elementary speed up for model checking synchronous perfect recall
We analyse the time complexity of the model checking problem for a logic of knowledge and past time in synchronous systems with perfect recall. Previously established bounds are k- exponential in the size of the system for specifications with k nested knowledge modalities.We show that the upper bound for positive (respectively, negative) specifications is polynomial (respectively, exponential) in the size of the system irrespective of the nesting depth
Computing Quantiles in Markov Reward Models
Probabilistic model checking mainly concentrates on techniques for reasoning
about the probabilities of certain path properties or expected values of
certain random variables. For the quantitative system analysis, however, there
is also another type of interesting performance measure, namely quantiles. A
typical quantile query takes as input a lower probability bound p and a
reachability property. The task is then to compute the minimal reward bound r
such that with probability at least p the target set will be reached before the
accumulated reward exceeds r. Quantiles are well-known from mathematical
statistics, but to the best of our knowledge they have not been addressed by
the model checking community so far.
In this paper, we study the complexity of quantile queries for until
properties in discrete-time finite-state Markov decision processes with
non-negative rewards on states. We show that qualitative quantile queries can
be evaluated in polynomial time and present an exponential algorithm for the
evaluation of quantitative quantile queries. For the special case of Markov
chains, we show that quantitative quantile queries can be evaluated in time
polynomial in the size of the chain and the maximum reward.Comment: 17 pages, 1 figure; typo in example correcte
A Temporal Logic for Hyperproperties
Hyperproperties, as introduced by Clarkson and Schneider, characterize the
correctness of a computer program as a condition on its set of computation
paths. Standard temporal logics can only refer to a single path at a time, and
therefore cannot express many hyperproperties of interest, including
noninterference and other important properties in security and coding theory.
In this paper, we investigate an extension of temporal logic with explicit path
variables. We show that the quantification over paths naturally subsumes other
extensions of temporal logic with operators for information flow and knowledge.
The model checking problem for temporal logic with path quantification is
decidable. For alternation depth 1, the complexity is PSPACE in the length of
the formula and NLOGSPACE in the size of the system, as for linear-time
temporal logic
Reasoning about Cardinal Directions between Extended Objects
Direction relations between extended spatial objects are important
commonsense knowledge. Recently, Goyal and Egenhofer proposed a formal model,
known as Cardinal Direction Calculus (CDC), for representing direction
relations between connected plane regions. CDC is perhaps the most expressive
qualitative calculus for directional information, and has attracted increasing
interest from areas such as artificial intelligence, geographical information
science, and image retrieval. Given a network of CDC constraints, the
consistency problem is deciding if the network is realizable by connected
regions in the real plane. This paper provides a cubic algorithm for checking
consistency of basic CDC constraint networks, and proves that reasoning with
CDC is in general an NP-Complete problem. For a consistent network of basic CDC
constraints, our algorithm also returns a 'canonical' solution in cubic time.
This cubic algorithm is also adapted to cope with cardinal directions between
possibly disconnected regions, in which case currently the best algorithm is of
time complexity O(n^5)
Analyzing the Interaction between Knowledge and Social Commitments in Multi-Agent Systems
Both knowledge and social commitments in Multi-Agent Systems (MASs) have long been under research independently, especially for agent communication. Plenty of work has been carried out to define their semantics. However, in concrete applications such as business settings and web-based applications, agents should reason about their knowledge and their social commitments at the same time, particularly when they are engaged in conversations. In fact, studying the interaction between knowledge and social commitments is still in its beginnings. Therefore, in this thesis, we aim to provide a practical and formal framework that analyzes the interaction between knowledge and communicative social commitments in MASs from the semantics, model checking, complexity, soundness and completeness perspectives.
To investigate such an interaction, we, first, combine CTLK (an extension of computation Tree Logic (CTL) with modality for reasoning about knowledge) and CTLC (an extension of CTL with modalities for reasoning about commitments and their fulfillments) in one new logic named CTLKC. By so doing, we identify some paradoxes in the new logic showing that simply combining current versions of commitment and knowledge logics results in a language of logic that violates some fundamental intuitions. Consequently, we propose CTLKC+, a new consistent logic of knowledge and commitments that fixes the identified paradoxes and allows us to reason about social commitments and knowledge simultaneously in a consistent manner. Second, we use correspondence theory for modal logics to prove the soundness and completeness of CTLKC+. To do so, we develop a set of reasoning postulates in CTLKC+ and correspond them to certain classes of frames. The existence of such correspondence allows us to prove that the logic generated by any subset of these postulates is sound and complete, with respect to the models that are based on the corresponding frames. Third, we address the problem of model checking CTLKC+ by transforming it to the problem of model checking GCTL∗ (a generalized version of Extended Computation Tree Logic (CTL∗) with action formulas) and ARCTL (the combination of CTL with action formulas) in order to respectively use the CWB-NC automata-based model checker and the extended NuSMV symbolic model checker. Moreover, we prove that the transformation techniques are sound. Fourth, we analyze the complexity of the proposed model checking techniques. The results of this analysis reveal that the complexity of our transformation procedures is PSPACE-complete for local concurrent programs with respect to the size of these programs and the length of the formula being checked. From the time perspective, we prove that the complexity of the proposed approaches is P-complete with regard to the size of the model and length of the formula. Finally, we implement our model checking approaches and report some experimental results by verifying the well-known NetBell payment protocol against some desirable properties
Model checking multi-agent systems
A multi-agent system (MAS) is usually understood as a system composed of interacting
autonomous agents. In this sense, MAS have been employed successfully as a modelling
paradigm in a number of scenarios, especially in Computer Science. However, the process
of modelling complex and heterogeneous systems is intrinsically prone to errors: for this
reason, computer scientists are typically concerned with the issue of verifying that a system
actually behaves as it is supposed to, especially when a system is complex.
Techniques have been developed to perform this task: testing is the most common technique,
but in many circumstances a formal proof of correctness is needed. Techniques
for formal verification include theorem proving and model checking. Model checking
techniques, in particular, have been successfully employed in the formal verification of
distributed systems, including hardware components, communication protocols, security
protocols.
In contrast to traditional distributed systems, formal verification techniques for MAS are
still in their infancy, due to the more complex nature of agents, their autonomy, and
the richer language used in the specification of properties. This thesis aims at making
a contribution in the formal verification of properties of MAS via model checking. In
particular, the following points are addressed:
• Theoretical results about model checking methodologies for MAS, obtained by
extending traditional methodologies based on Ordered Binary Decision Diagrams (OBDDS) for temporal logics to multi-modal logics for time, knowledge, correct behaviour, and strategies of agents. Complexity results for model checking these logics
(and their symbolic representations).
• Development of a software tool (MCMAS) that permits the specification and verification
of MAS described in the formalism of interpreted systems.
• Examples of application of MCMAS to various MAS scenarios (communication, anonymity, games, hardware diagnosability), including experimental results, and comparison with other tools available
Probabilistic and Epistemic Model Checking for Multi-Agent Systems
Model checking is a formal technique widely used to verify security and communication protocols in epistemic multi-agent systems against given properties. Qualitative
properties such as safety and liveliness have been widely analysed in the literature. However, systems also have quantitative and uncertain (i.e., probabilistic) properties such as degree of reliability and reachability, which still need further attention from the model checking perspective. In this dissertation, we analyse such properties and present a new method for probabilistic model checking of epistemic multi-agent
systems specified by a new probabilistic-epistemic logic PCTLK. We model multiagent systems distributed knowledge bases using probabilistic interpreted systems. We also define transformations from those interpreted systems into discrete-time Markov chains and from PCTLK formulae to PCTL formulae, an existing extension of CTL with probabilities. By so doing, we are able to convert the PCTLK model checking problem into the PCTL one. We address the problem of verifying probabilistic properties
and epistemic properties in concurrent probabilistic systems as well. We then prove that model checking a formula of PCTLK in concurrent probabilistic systems is
PSPACE-complete. Furthermore, we represent models associated with PCTLK logic symbolically with Multi-Terminal Binary Decision Diagrams (MTBDDs).
Finally, we make use of PRISM, the model checker of PCTL without adding new computation cost. Dining cryptographers protocol is implemented to show the
applicability of the proposed technique along with performance analysis and comparison in terms of execution time and state space scalability with MCK, an existing
epistemic-probabilistic model checker, and MCMAS, a model checker for multi-agent systems. Another example, NetBill protocol, is also implemented with PRISM to verify probabilistic epistemic properties and to evaluate the complexity of this verification
Model Checking Social Network Models
A social network service is a platform to build social relations among people
sharing similar interests and activities. The underlying structure of a social
networks service is the social graph, where nodes represent users and the arcs
represent the users' social links and other kind of connections. One important
concern in social networks is privacy: what others are (not) allowed to know
about us. The "logic of knowledge" (epistemic logic) is thus a good formalism
to define, and reason about, privacy policies. In this paper we consider the
problem of verifying knowledge properties over social network models (SNMs),
that is social graphs enriched with knowledge bases containing the information
that the users know. More concretely, our contributions are: i) We prove that
the model checking problem for epistemic properties over SNMs is decidable; ii)
We prove that a number of properties of knowledge that are sound w.r.t. Kripke
models are also sound w.r.t. SNMs; iii) We give a satisfaction-preserving
encoding of SNMs into canonical Kripke models, and we also characterise which
Kripke models may be translated into SNMs; iv) We show that, for SNMs, the
model checking problem is cheaper than the one based on standard Kripke models.
Finally, we have developed a proof-of-concept implementation of the
model-checking algorithm for SNMs.Comment: In Proceedings GandALF 2017, arXiv:1709.0176
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