Large Pseudo-Counts and L2L_2-Norm Penalties Are Necessary for the Mean-Field Inference of Ising and Potts Models

Mean field (MF) approximation offers a simple, fast way to infer direct interactions between elements in a network of correlated variables, a common, computationally challenging problem with practical applications in fields ranging from physics and biology to the social sciences. However, MF methods achieve their best performance with strong regularization, well beyond Bayesian expectations, an empirical fact that is poorly understood. In this work, we study the influence of pseudo-count and $L_2$-norm regularization schemes on the quality of inferred Ising or Potts interaction networks from correlation data within the MF approximation. We argue, based on the analysis of small systems, that the optimal value of the regularization strength remains finite even if the sampling noise tends to zero, in order to correct for systematic biases introduced by the MF approximation. Our claim is corroborated by extensive numerical studies of diverse model systems and by the analytical study of the $m$-component spin model, for large but finite $m$. Additionally we find that pseudo-count regularization is robust against sampling noise, and often outperforms $L_2$-norm regularization, particularly when the underlying network of interactions is strongly heterogeneous. Much better performances are generally obtained for the Ising model than for the Potts model, for which only couplings incoming onto medium-frequency symbols are reliably inferred.Comment: 25 pages, 17 figure

Casimir-Polder forces, boundary conditions and fluctuations

We review different aspects of the atom-atom and atom-wall Casimir-Polder forces. We first discuss the role of a boundary condition on the interatomic Casimir-Polder potential between two ground-state atoms, and give a physically transparent interpretation of the results in terms of vacuum fluctuations and image atomic dipoles. We then discuss the known atom-wall Casimir-Polder force for ground- and excited-state atoms, using a different method which is also suited for extension to time-dependent situations. Finally, we consider the fluctuation of the Casimir-Polder force between a ground-state atom and a conducting wall, and discuss possible observation of this force fluctuation.Comment: 5 page

Multi-wavelength visibility measurements of the red giant R Doradus

We present visibility measurements of the nearby Mira-like star R Doradus taken over a wide range of wavelengths (650--990 nm). The observations were made using MAPPIT (Masked APerture-Plane Interference Telescope), an interferometer operating at the 3.9-m Anglo-Australian Telescope. We used a slit to mask the telescope aperture and prism to disperse the interference pattern in wavelength. We observed in R Dor strong decreases in visibility within the TiO absorption bands. The results are in general agreement with theory but differ in detail, suggesting that further work is needed to refine the theoretical models.Comment: 8 pages; SPIE Conf. 4006 "Interferometry in Optical Astronomy

Possibilities for pedagogy in Further Education: Harnessing the abundance of literacy

In this report, it is argued that the most salient factor in the contemporary communicative landscape is the sheer abundance and diversity of possibilities for literacy, and that the extent and nature of students' communicative resources is a central issue in education. The text outlines the conceptual underpinnings of the Literacies for Learning in Further Education project in a social view of literacy, and the associated research design, methodology and analytical framework. It elaborates on the notion of the abundance of literacies in students' everyday lives, and on the potential for harnessing these as resources for the enhancement of learning. It provides case studies of changes in practice that have been undertaken by further education staff in order to draw upon students' everyday literacy practices on Travel and Tourism and Multimedia courses. It ends with some of the broad implications for conceptualising learning that arise from researching through the lens of literacy practices

Hysterectomy, endometrial ablation, and levonorgestrel releasing intrauterine system (Mirena) for treatment of heavy menstrual bleeding : cost effectiveness analysis

Peer reviewedPublisher PD

Search for correlation effects in linear chains of trapped ions

We report a precise search for correlation effects in linear chains of 2 and 3 trapped Ca+ ions. Unexplained correlations in photon emission times within a linear chain of trapped ions have been reported, which, if genuine, cast doubt on the potential of an ion trap to realize quantum information processing. We observe quantum jumps from the metastable 3d 2D_{5/2} level for several hours, searching for correlations between the decay times of the different ions. We find no evidence for correlations: the number of quantum jumps with separations of less than 10 ms is consistent with statistics to within errors of 0.05%; the lifetime of the metastable level derived from the data is consistent with that derived from independent single-ion data at the level of the experimental errors 1%; and no rank correlations between the decay times were found with sensitivity to rank correlation coefficients at the level of |R| = 0.024.Comment: With changes to introduction. 5 pages, including 4 figures. Submitted to Europhys. Let

Theoretical model for the images formed by a spherical particle in a coherent imaging system: comparison

A simple theoretical model is presented that allows calculation of the image produced by a spherical absorbing particle illuminated by monochromatic, coherent laser light. Results presented in this paper are restricted to a singe-lens imaging system, although generalization to more complex imaging system configurations would be straightfoward. The method uses classic Lorenz-Mie scattering theory to obtain the electromagnetic field external to an absorbing spherical particle and a Fourier optics approach to calculate the intensities in the image plane. Experimental results evaluating focus characteristics are examined for 50 um diameter water droplets using an N2 laser imaging system in conjunction with a digital image processor, and the experimental images are compared with a digital image processor, and the experimental images are compared to the results of the theoretical model. Comparative focus criteria results are particularly useful in aerosol science research involving dynamic particle size measuremetns in which criteria for focus and depth of field must be established