Distributed Consensus, Revisited

We provide a novel model to formalize a well-known algorithm, by Chandra and Toueg, that solves Consensus among asynchronous distributed processes in the presence of a particular class of failure detectors (Diamond S or, equivalently, Omega), under the hypothesis that only a minority of processes may crash. The model is defined as a global transition system that is unambigously generated by local transition rules. The model is syntax-free in that it does not refer to any form of programming language or pseudo code. We use our model to formally prove that the algorithm is correct

The Period Changes of the Cepheid RT Aurigae

Observations of the light curve for the 3.7-day Cepheid RT Aur both before and since 1980 indicate that the variable is undergoing an overall period increase, amounting to +0.082 +-0.012 s/yr, rather than a period decrease, as implied by all observations prior to 1980. Superposed on the star's O-C variations is a sinusoidal trend that cannot be attributed to random fluctuations in pulsation period. Rather, it appears to arise from light travel time effects in a binary system. The derived orbital period for the system is P = 26,429 +-89 days (72.36 +-0.24 years). The inferred orbital parameters from the O-C residuals differ from those indicated by existing radial velocity data. The latter imply the most reasonable results, namely a1 sin i = 9.09 (+-1.81) x 10^8 km and a minimum secondary mass of M2 = 1.15 +-0.25 Msun. Continued monitoring of the brightness and radial velocity changes in the Cepheid are necessary to confirm the long-term trend and to provide data for a proper spectroscopic solution to the orbit.Comment: Accepted for publication in PASP (November 2007

O expletivo ele em domínios dependentes em português europeu

Tese de mestrado, Linguística, Universidade de Lisboa, Faculdade de Letras, 2013Esta dissertação tem como objeto de estudo a distribuição do expletivo ele nãopadrão em domínios oracionais dependentes em português europeu. Partindo de uma caracterização do expletivo ele como elemento linguístico relacionado com posições sintáticas na periferia esquerda da frase, a investigação aqui apresentada centra-se sobre a descrição sintática dos contextos dependentes que envolvem este expletivo, a fim de verificar: a posição ocupada por ele na periferia esquerda de domínios oracionais nãomatriz, a ocorrência de simetrias/assimetrias na distribuição e nos efeitos discursivos do expletivo em orações dependentes relativamente às propriedades atribuídas a este expletivo em orações principais. Neste sentido, a presente investigação visa também contribuir para o estudo comparativo da estrutura da periferia esquerda em frases principais e em frases dependentes. A base empírica aqui apresentada incide sobre dados de variedades não-padrão do português europeu recolhidos do Corpus Dialectal para o Estudo da Sintaxe (CORDIAL-SIN), desenvolvido pelo Centro de Linguística da Universidade de Lisboa. No plano da distribuição sintática, em orações dependentes, dois tipos de expletivo são distinguidos: o expletivo pré-verbal, que ocorre em posição pré ou pósconector e que pode ser associado a uma projeção ForceP na periferia esquerda da estrutura frásica, ou no domínio da oração dependente ou no domínio matriz de uma oração dependente periférica; pontualmente, o expletivo pós-verbal, que aparece em uma posição mais baixa. A presença do expletivo ele em contextos não-matriz manifesta propriedades discursivas enfáticas relevantes para a codificação de efeitos ilocutórios, em muitos dos exemplos analisados associáveis a domínios dependentes assertivos.Abstract The object of the study in this dissertation consists of the distribution of nonstandard expletive ele in dependent clausal domains in European Portuguese. Based on a characterization of the expletive ele as a linguistic element related to syntactic positions on the left periphery of the sentence, the research presented here focuses on the syntactical description of dependents contexts involving this expletive, in order to examine: the position occupied by ele in left periphery of non matrix clausal domains, the occurrence of symmetry/asymmetry in the distribution and discursive effects of the expletive in dependent clauses regarding the properties attributed to this expletive in main clauses. In this sense, this research also aims to contribute to the comparative study of the left periphery structure in main clauses and dependents phrases. The empirical study presented here focuses on non-standard European Portuguese data collected from the Syntax-oriented Corpus of Portuguese Dialects (CORDIAL-SIN), developed by the Centro de Linguística da Universidade de Lisboa. In terms of syntactic distribution, in dependents clauses, two expletive types are distinguished: the preverbal expletive, which occurs in a pre or post-connector position and can be associated with a ForceP projection in the left periphery of the sentence, either in dependents clauses or in the matrix clause domain of a peripheral dependent clause; and, occasionally, a post verbal expletive, which appears in a lower position. The presence of the expletive ele in non-matrix contexts shows discursive properties relevant to codify illocutionary effects, and in many examples it can be associated with assertive dependents domains

The first census of homeless persons in Ireland.

Contains fulltext : 33039.pdf (preprint version ) (Open Access

2006 European report on the drugs problem.

The model of probabilistic I/O automata of Segala and Lynch is used for the formal speci cation and analysis of the root contention protocol from the physical layer of the IEEE 1394 ("FireWire") standard. In our mode

Modeling consensus in a process calculus

Abstract. We give a process calculus model that formalizes a wellknown algorithm (introduced by Chandra and Toueg) solving consensus in the presence of a particular class of failure detectors (♦S); we use our model to formally prove that the algorithm satisfies its specification.

Computer-Assisted Simulation Proofs

This paper presents a scalable approach to reasoning formally about distributed algorithms. It uses results about I/O automata to extract a set of proof obligations for showing that the behaviors of one algorithm are among those of another, and it uses the Larch tools for specification and deduction to discharge these obligations in a natural and easy-to-read fashion. The approach is demonstrated by proving the behavior equivalence of two high-level specifications for a communication protocol

Randomized Dining Philosophers Without Fairness Assumption

We consider Lehmann-Rabin's randomized solution to the well-known problem of the dining philosophers. Up to now, such an analysis has always required a "fairness" assumption on the scheduler: if a philosopher is continuously hungry then he must eventually be scheduled. In contrast here, we modify the algorithm in order to get rid of the fairness assumption. We claim that the spirit of the original algorithm is preserved. We prove that, for any (possibly unfair) scheduler, the modified algorithm converges: every computation reaches with probability 1 a configuration where some philosopher eats. Furthermore, we are now able to evaluate the expected time of convergence as a number of transitions. We show that, for some "malicious" scheduler, this expected time is at least exponential in the number N of philosophers