Walker Evans' American Photographs

Abstract

It is an article of faith among photographers and scholars that Walker Evans' American Photographs is a sequence of pictures rather than a simple collection or anthology; that is to say that the photographs should be looked at in the order in which they are given. Indeed the book is often spoken of as innovative and influential in precisely that respect (among others). Some years ago I set out to learn more about the origin of Evans' sequence in the context of a broader investigation of American Photographs.¹ That study (which remained unpublished) has now been superseded in many ways by Alan Trachtenberg's insightful discussion in the book Reading American Photographs, which appeared in 1989.² Nevertheless some of my original material remains useful and I offer it here largely intact, with only a small number of revisions and additions.     

Synthesis and Photocatalytic Properties of Single Crystalline (Ga1-xZnx)(N1-xOx) Nanotubes

Recently, (Ga_1-xZn_x)(N_1-xO_x) has gained widespread attention as a comparatively high efficiency photocatalyst for visible-light-driven overall water splitting. Despite significant gains in efficiency over the past several years, a majority of the photogenerated carriers recombine within bulk powders. To improve the photocatalytic activity, we used an epitaxial casting method to synthesize single-crystalline, high surface area (Ga_1-xZn_x)(N_1-xO_x) nanotubes with ZnO compositions up to x=0.10. Individual nanotubes showed improved homogeneity over powder samples due to a well defined epitaxial interface for ZnO diffusion into GaN. Absorption measurements showed that the ZnO incorporation shifts the absorption into the visible region with a tail out to 500 nm. Gas chromatography (GC) was used to compare the solar water splitting activity of (Ga_1-xZn_x)(N_1-xO_x) nanotubes (x=0.05–0.10) with similar composition powders. Cocatalyst decorated samples were dispersed in aqueous solutions of CH₃OH and AgO₂CCH₃ to monitor the H⁺ reduction and H₂O oxidation half reactions, respectively. The nanotubes were found to have approximately 1.5–2 times higher photocatalytic activity than similar composition powders for the rate limiting H⁺ reduction half reaction. These results demonstrate that improvements in homogeneity and surface area using the nanotube geometry can enhance the photocatalytic activity of GaN:ZnO for solar water splitting.     

Building test cases and oracles to automate the testing of web database applications

Many organizations rely on web applications that use back-end databases to store important data. Testing such applications requires significant effort. Manual testing alone is often impractical, so testers also rely on automated testing techniques. However, current automated testing techniques may produce false positives (or false negatives) even in a perfectly working system because the outcome of a test case depends on the state of the database which changes over time as data is inserted and deleted. The Automatic Database Tester (AutoDBT) generates functional test cases that account for database updates. AutoDBT takes as input a model of the application and a set of testing criteria. The model consists of a state transition diagram that shows how users navigate pages, a data specification that captures how data flows, and an update specification that shows how the database is updated. AutoDBT generates guard queries to determine whether the database is in a state conducive to performing and evaluating tests. AutoDBT also generates partial oracles to help validate whether a back-end database is updated correctly during testing. This paper describes the design of AutoDBT, a prototype implementation, several experiments with the prototype, and four case studies.     

Stochastic Geometry and Random Graphs for the Analysis and Design of Wireless Networks

Wireless networks are fundamentally limited by the intensity of the received signals and by their interference. Since both of these quantities depend on the spatial location of the nodes, mathematical techniques have been developed in the last decade to provide communication-theoretic results accounting for the network?s geometrical configuration. Often, the location of the nodes in the network can be modeled as random, following for example a Poisson point process. In this case, different techniques based on stochastic geometry and the theory of random geometric graphs ? including point process theory, percolation theory, and probabilistic combinatorics ? have led to results on the connectivity, the capacity, the outage probability, and other fundamental limits of wireless networks. This tutorial article surveys some of these techniques, discusses their application to model wireless networks, and presents some of the main results that have appeared in the literature. It also serves as an introduction to the field for the other papers in this special issue.     

3D Simulations to investigate initial condition effects on the growth of Rayleigh–Taylor mixing

The effect of initial conditions on the growth rate of turbulent Rayleigh–Taylor (RT) mixing has been studied using carefully formulated numerical simulations. An implicit large-eddy simulation (ILES) that uses a finite-volume technique was employed to solve the three-dimensional incompressible Euler equations with numerical dissipation. The initial conditions were chosen to test the dependence of the RT growth parameters (α_b, α_s) on variations in (a) the spectral bandwidth, (b) the spectral shape, and (c) discrete banded spectra. Our findings support the notion that the overall growth of the RT mixing is strongly dependent on initial conditions. Variation in spectral shapes and bandwidths are found to have a complex effect of the late time development of the RT mixing layer, and raise the question of whether we can design RT transition and turbulence based on our choice of initial conditions. In addition, our results provide a useful database for the initialization and development of closures describing RT transition and turbulence.     

Mode Interactions in Four-frequency Degeneracy-lifted Ring Lasers

Abstract

A non-planar ring cavity, with a non-reciprocal bias in the form of the longitudinal Zeeman effect on the gain medium, has been used to demonstrate four-mode bidirectional operation in a He-Ne ring laser at 632·8 nm. It has also been shown that this ring system is sensitive to rotation.     

Sense induction in folksonomies: a review

Folksonomies, often known as tagging systems, such as the ones used on the popular Delicious or Flickr websites, use a very simple Knowledge Organisation System. Users have thus been quick to adopt this system and create extensive annotations on the Web. However, because of the simplicity of the folksonomy model, the semantics of the tags used is not explicit and can only be inferred from their context of use. This is a barrier for the automatic use of such Knowledge Organisation Systems by computers and new techniques have been developed to extract the semantic of the tags. In this article we discuss the drawbacks of some of these approaches and propose a generalization of the different approaches to detect new senses of terms in a folksonomy. Another weak point of the current state of the art in the field is the lack of formal evaluation methodology; we thus propose a novel evaluation framework. We introduce a dataset and evaluation methodology that enable the comparison of results between different approaches to sense induction in folksonomies. Finally we discuss the performances of different approaches to the task of homonymous/polysemous tag detection and synonymous identification.     

A cost-benefit analysis of increased investment in Melbourne's public transport system

Abstract

The paper considers economic effects of increased investment in public transport in Melbourne, based on investment plans of VicRail and the M.M.T.B. over a 5 year period, 1981–5. If the investment were to encourage a shift in patronage from private to public transport, this would change the magnitude of costs associated with passenger transport in Melbourne. Calculations are made of these possible cost savings, given particular levels of shift in patronage. The paper concludes by reviewing the feasibility of achieving given patronage changes.