Elucidating concurrent algorithms via layers of abstraction and reification

Arguing that intricate concurrent programs satisfy their specifications can be difficult; recording understandable explanations is important for subsequent readers. Abstraction is a key tool even for sequential programs. The purpose here is to explore some abstractions that help readers (and writers) understand the design of concurrent programs. As an illustration, the paper presents a formal development of a non-trivial parallel program: Simpson’s implementation of asynchronous communication mechanisms. Although the correctness of this “4-slot algorithm” has been shown elsewhere, earlier proofs fail to offer much insight into the design. From an understandable (yet formal) design history of this one algorithm, the techniques employed in the explanation are teased out for wider application. Among these techniques is using a “fiction of atomicity” as an aid to understanding the initial steps of development. The rely-guarantee approach is, here, combined with notions of read/write frames and “phased” specifications; furthermore, the atomicity assumptions implied by the rely/guarantee conditions are achieved by clever choice of data representations.     

A Structural Proof of the Soundness of Rely/guarantee Rules

Various forms of rely/guarantee conditions have been used torecord and reason about interference in ways that provide compositionaldevelopment methods for concurrent programs. This article illustratessuch a set of rules and proves their soundness. The underlyingconcurrent language allows fine-grained interleaving and nestedconcurrency; it is defined by an operational semantics; theproof that the rely/guarantee rules are consistent with thatsemantics (including termination) is by a structural induction.A key lemma which relates the states which can arise from theextra interference that results from taking a portion of theprogram out of context makes it possible to do the proofs withouthaving to perform induction over the computation history. Thislemma also offers a way to think about expressibility issuesaround auxiliary variables in rely/guarantee conditions.     

Latent Users in an Online User-Generated Content Community

Online communities depend on the persistent contributions of heterogeneous users with diverse motivations and ways of participating. As these online communities exist over time, it is possible that users change the way in which they contribute to the site. Through interviews with 31 long-term members of a user-generated content community who have decreased their participation on the site, we examined the meaning that these users gave to their contribution and how their new participation patterns related to their initial motivations. We complement the reader-to-leader framework (Preece and Shneiderman: AIS Transactions on Human-Computer Interaction, vol. 1, no. 1, pp. 13–32, 2009) by propounding the concept of latent user to understand decreasing content contribution and user life-cycles in online communities. We showed that even though latent users decrease their content contribution, their participation becomes more selective and remained consistent with initial motivations to participate.     

Cultivating Social Resources on Social Network Sites: Facebook Relationship Maintenance Behaviors and Their Role in Social Capital Processes

This study explores the relationship between perceived bridging social capital and specific Facebook‐enabled communication behaviors using survey data from a sample of U.S. adults (N=614). We explore the role of a specific set of Facebook behaviors that support relationship maintenance and assess the extent to which demographic variables, time on site, total and “actual” Facebook Friends, and this new measure (Facebook Relationship Maintenance Behaviors) predict bridging social capital. Drawing upon scholarship on social capital and relationship maintenance, we discuss the role of social grooming and attention‐signaling activities in shaping perceived access to resources in one's network as measured by bridging social capital.     

A lower bound damage-based finite element simulation of stretch flange forming of Al–Mg alloys

Numerical simulation of stretch flange forming of Al–Mg sheet AA5182 was performed using the upper and lower bound constitutive models of Gurson–Tvergaard–Needleman (GTN) and Sun and Wang, respectively. Stress and strain-controlled nucleation rules are adopted for both models. The lower bound model of Sun and Wang has been extended to include the void coalescence criterion of Tvergaard and Needleman to form the so-called Sun–Tvergaard–Needleman (STN) model. Upper and lower bound formability predictions are combined to create a predictive formability band as actual formability lies between these limits. The resulting formability predictions are compared with experimental results and an appropriate void nucleation stress and strain suggested.     

Summary from the epistemic uncertainty workshop: consensus amid diversity

The ‘Epistemic Uncertainty Workshop’ sponsored by Sandia National Laboratories was held in Albuquerque, New Mexico, on 6–7 August 2002. The workshop was organized around a set of Challenge Problems involving both epistemic and aleatory uncertainty that the workshop participants were invited to solve and discuss. This concluding article in a special issue of Reliability Engineering and System Safety based on the workshop discusses the intent of the Challenge Problems, summarizes some discussions from the workshop, and provides a technical comparison among the papers in this special issue. The Challenge Problems were computationally simple models that were intended as vehicles for the illustration and comparison of conceptual and numerical techniques for use in analyses that involve: (i) epistemic uncertainty, (ii) aggregation of multiple characterizations of epistemic uncertainty, (iii) combination of epistemic and aleatory uncertainty, and (iv) models with repeated parameters. There was considerable diversity of opinion at the workshop about both methods and fundamental issues, and yet substantial consensus about what the answers to the problems were, and even about how each of the four issues should be addressed. Among the technical approaches advanced were probability theory, Dempster–Shafer evidence theory, random sets, sets of probability measures, imprecise coherent probabilities, coherent lower previsions, probability boxes, possibility theory, fuzzy sets, joint distribution tableaux, polynomial chaos expansions, and info-gap models. Although some participants maintained that a purely probabilistic approach is fully capable of accounting for all forms of uncertainty, most agreed that the treatment of epistemic uncertainty introduces important considerations and that the issues underlying the Challenge Problems are legitimate and significant. Topics identified as meriting additional research include elicitation of uncertainty representations, aggregation of multiple uncertainty representations, dependence and independence, model uncertainty, solution of black-box problems, efficient sampling strategies for computation, and communication of analysis results.