An Optimal Self-Stabilizing Firing Squad

Consider a fully connected network where up to $t$ processes may crash, and all processes start in an arbitrary memory state. The self-stabilizing firing squad problem consists of eventually guaranteeing simultaneous response to an external input. This is modeled by requiring that the non-crashed processes "fire" simultaneously if some correct process received an external "GO" input, and that they only fire as a response to some process receiving such an input. This paper presents FireAlg, the first self-stabilizing firing squad algorithm. The FireAlg algorithm is optimal in two respects: (a) Once the algorithm is in a safe state, it fires in response to a GO input as fast as any other algorithm does, and (b) Starting from an arbitrary state, it converges to a safe state as fast as any other algorithm does.Comment: Shorter version to appear in SSS0

Natural Theories of Ultra-Low Mass PNGB's: Axions and Quintessence

We consider the Wilson Line PNGB which arises in a U(1)^N gauge theory, abstracted from a latticized, periodically compactified extra dimension U(1). Planck scale breaking of the PNGB's global symmetry is suppressed, providing natural candidates for the axion and quintessence. We construct an explicit model in which the axion may be viewed as the 5th component of the U(1)_Y gauge field in a 1+4 latticized periodically compactified extra dimension. We also construct a quintessence PNGB model where the ultra-low mass arises from Planck-scale suppressed physics itself.Comment: 20 pages, fixed typo and reference

Schooling for conflict transformation : a case study from Northern Uganda

Civil wars impede progress towards the Millennium Development Goals. As many conflicts erupt within a short time, it is important to know what may increase the chances of sustainable peace. Access to education is a factor but relatively little is known about the contribution of what students learn in school. This thesis aims to respond to a research gap by addressing the foll owing question: 'How can schooling contribute to conflict transformation?' Significant curricular approaches that may be used after civil war - peace education, human rights education and citizenship education - are assessed for their strengths and weaknesses. As no single approach is found to be sufficient for conflict transformation, a framework is proposed based on three fundamental concepts: (i) truth seeking; (ii) reconciliation; and (iii) inclusive citizenship. This framework is examined through a qualitative case study of curriculum in seven schools in a district in northern Uganda that is emerging from a twenty-year civil war. The curriculum of four primary schools, two secondary schools, one special school and one teacher training college was studied over a three-month period. A structure of knowledge, skills and values was used to research the framework at a detailed level. It is found that schools exhibit good socialization of reconciliation values and some development of problem-solving and communication skills. There is some understanding of human rights, but little knowledge of history, or of local, national and international political/legal systems. There is minimal development of discussion and critical thinking skills. It is argued that the framework can be used to investigate other schools and to inform the design of a curriculum that can contribute to conflict transformation, with the ultimate aim of reducing the risk of civil war re-eruption

Threats to the validity of the Collegiate Learning Assessment (CLA+) as a measure of critical thinking skills and implications for Learning Gain

The University of Reading Learning Gain project is a three-year longitudinal project to test and evaluate a range of available methodologies and to draw conclusions on what might be the right combination of instruments for the measurement of Learning Gain in higher education. This paper analyses the validity of a measure of critical thinking skills, the Collegiate Learning Assessment (CLA+) and the implications of using this standardised test as a proxy for Learning Gain. The paper reviews five inferences regarding the interpretations and use of test scores: construct representation, scoring, generalisation, extrapolation and decision-making. Each section reviews some of the available evidence in support of the claims the CLA+ makes and the threats to their validity. The possible impact of these issues on Learning Gain in the UK is considered

Six Years of Chandra Observations of Supernova Remnants

We present a review of the first six years of Chandra X-ray Observatory observations of supernova remnants. From the official "first-light" observation of Cassiopeia A that revealed for the first time the compact remnant of the explosion, to the recent million-second spectrally-resolved observation that revealed new details of the stellar composition and dynamics of the original explosion, Chandra observations have provided new insights into the supernova phenomenon. We present an admittedly biased overview of six years of these observations, highlighting new discoveries made possible by Chandra's unique capabilities.Comment: 82 pages, 28 figures, for the book Astrophysics Update

From Denotational to Operational and Axiomatic Semantics for ALGOL-like Languages: An Overview

The advantages of denotational over operational semantics are argued. A denotational semantics is provided for an ALGOL-like language with finite-model procedures, blocks with local storage, and sharing (aliasing). Procedure declarations are completely explained in the ususal framework of complete partial orders, but cpo's are inadequate for the semantics of blocks, and a new class of store models is developed. Partial correctness theory over store models is developed for commands which may contain calls to global procedures, but do not contain function procedures returning storable values