Stop It, and Be Stubborn!

A system is AG EF terminating, if and only if from every reachable state, a terminal state is reachable. This publication argues that it is beneficial for both catching non-progress errors and stubborn set state space reduction to try to make verification models AG EF terminating. An incorrect mutual exclusion algorithm is used as an example. The error does not manifest itself, unless the first action of the customers is modelled differently from other actions. An appropriate method is to add an alternative first action that models the customer stopping for good. This method typically makes the model AG EF terminating. If the model is AG EF terminating, then the basic strong stubborn set method preserves safety and some progress properties without any additional condition for solving the ignoring problem. Furthermore, whether the model is AG EF terminating can be checked efficiently from the reduced state space

Belief Dynamics in Social Networks: A Fluid-Based Analysis

The advent and proliferation of social media have led to the development of mathematical models describing the evolution of beliefs/opinions in an ecosystem composed of socially interacting users. The goal is to gain insights into collective dominant social beliefs and into the impact of different components of the system, such as users' interactions, while being able to predict users' opinions. Following this thread, in this paper we consider a fairly general dynamical model of social interactions, which captures all the main features exhibited by a social system. For such model, by embracing a mean-field approach, we derive a diffusion differential equation that represents asymptotic belief dynamics, as the number of users grows large. We then analyze the steady-state behavior as well as the time dependent (transient) behavior of the system. In particular, for the steady-state distribution, we obtain simple closed-form expressions for a relevant class of systems, while we propose efficient semi-analytical techniques in the most general cases. At last, we develop an efficient semi-analytical method to analyze the dynamics of the users' belief over time, which can be applied to a remarkably large class of systems.Comment: submitted to IEEE TNS

The False Promise of Thought Experimentation in Moral and Political Philosophy

Prof. Miščević has long been an ardent defender of the use of thought experiments in philosophy, foremost metaphysics, epistemology and philosophy of mind. Recently he has, in his typically sophisticated manner, extended his general account of philosophical thought-experimenting to the domain of normative politics. Not only can the history of political philosophy be better understood and appreciated, according to Miščević, when seen as a more or less continuous, yet covert, practice of thought-experimenting, the very progress of the discipline may crucially depend on finding the right balance between the constraints of (biological, psychological, economic, political, and so on) reality and political-moral ideals when we set to design our basic political notions and institutions. I have much less confidence in this project than prof. Miščević does. As a subspecies of moral TE, political TE share all their problems plus exhibit some of their own. In the paper, I present and discuss two types of evidence that threaten to undermine political philosophers’ trust in thought-experiments and the ethical/political intuitions elicited by them: (i) the dismal past record of thought-experimentation in moral and political philosophy; and (ii) the variety, prevalence, and stubbornness, of bias in ordinary social/political judgment

Supporting Domain-Specific State Space Reductions through Local Partial-Order Reduction

Model checkers offer to automatically prove safety and liveness properties of complex concurrent software systems, but they are limited by state space explosion. Partial-Order Reduction (POR) is an effective technique to mitigate this burden. However, applying existing notions of POR requires to verify conditions based on execution paths of unbounded length, a difficult task in general. To enable a more intuitive and still flexible application of POR, we propose local POR (LPOR). LPOR is based on the existing notion of statically computed stubborn sets, but its locality allows to verify conditions in single states rather than over long paths. As a case study, we apply LPOR to message-passing systems. We implement it within the Java Pathfinder model checker using our general Java-based LPOR library. Our experiments show significant reductions achieved by LPOR for model checking representative message-passing protocols and, maybe surprisingly, that LPOR can outperform dynamic POR. © 2011 IEEE

Selected Essays on the Conflict of Laws. By Brainerd Currie.

Most subspace identication algorithms are not applicable to closed-loop identication because they require future input to be uncorrelated with pastinnovation. In this paper, we propose a new subspace identication method that remove this requirement by using a parsimonious model formulation with innovation estimation. A simulation example is included to show the effectiveness of the proposed method

The mutable consensus protocol

In this paper we propose the mutable consensus protocol, a pragmatic and theoretically appealing approach to enhance the performance of distributed consensus. First, an apparently inefficient protocol is developed using the simple stubborn channel abstraction for unreliable message passing. Then, performance is improved by introducing judiciously chosen finite delays in the implementation of channels. Although this does not compromise correctness, which rests on an asynchronous system model, it makes it likely that the transmission of some messages is avoided and thus the message exchange pattern at the network level changes noticeably. By choosing different delays in the underlying stubborn channels, the mutable consensus protocol can actually be made to resemble several different protocols. Besides presenting the mutable consensus protocol and four different mutations, we evaluate in detail the particularly interesting permutation gossip mutation, which allows the protocol to scale gracefully to a large number of processes by balancing the number of messages to be handled by each process with the number of communication steps required to decide. The evaluation is performed using a realistic simulation model which accurately reproduces resource consumption in real systems.FCT , project STRONGREP (POSI/CHS/41285/2001)

Parent Resource Packet - A Guide for New Parents

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