On the Chromatic Thresholds of Hypergraphs

Let F be a family of r-uniform hypergraphs. The chromatic threshold of F is the infimum of all non-negative reals c such that the subfamily of F comprising hypergraphs H with minimum degree at least $c \binom{|V(H)|}{r-1}$ has bounded chromatic number. This parameter has a long history for graphs (r=2), and in this paper we begin its systematic study for hypergraphs. {\L}uczak and Thomass\'e recently proved that the chromatic threshold of the so-called near bipartite graphs is zero, and our main contribution is to generalize this result to r-uniform hypergraphs. For this class of hypergraphs, we also show that the exact Tur\'an number is achieved uniquely by the complete (r+1)-partite hypergraph with nearly equal part sizes. This is one of very few infinite families of nondegenerate hypergraphs whose Tur\'an number is determined exactly. In an attempt to generalize Thomassen's result that the chromatic threshold of triangle-free graphs is 1/3, we prove bounds for the chromatic threshold of the family of 3-uniform hypergraphs not containing {abc, abd, cde}, the so-called generalized triangle. In order to prove upper bounds we introduce the concept of fiber bundles, which can be thought of as a hypergraph analogue of directed graphs. This leads to the notion of fiber bundle dimension, a structural property of fiber bundles that is based on the idea of Vapnik-Chervonenkis dimension in hypergraphs. Our lower bounds follow from explicit constructions, many of which use a hypergraph analogue of the Kneser graph. Using methods from extremal set theory, we prove that these Kneser hypergraphs have unbounded chromatic number. This generalizes a result of Szemer\'edi for graphs and might be of independent interest. Many open problems remain.Comment: 37 pages, 4 figure

Hepatoma cell density promotes claudin-1 and scavenger receptor BI expression and hepatitis C virus internalization.

Hepatitis C virus (HCV) entry occurs via a pH- and clathrin-dependent endocytic pathway and requires a number of cellular factors, including CD81, the tight-junction proteins claudin 1 (CLDN1) and occludin, and scavenger receptor class B member I (SR-BI). HCV tropism is restricted to the liver, where hepatocytes are tightly packed. Here, we demonstrate that SR-BI and CLDN1 expression is modulated in confluent human hepatoma cells, with both receptors being enriched at cell-cell junctions. Cellular contact increased HCV pseudoparticle (HCVpp) and HCV particle (HCVcc) infection and accelerated the internalization of cell-bound HCVcc, suggesting that the cell contact modulation of receptor levels may facilitate the assembly of receptor complexes required for virus internalization. CLDN1 overexpression in subconfluent cells was unable to recapitulate this effect, whereas increased SR-BI expression enhanced HCVpp entry and HCVcc internalization, demonstrating a rate-limiting role for SR-BI in HCV internalization

Light Scattering by Pure Seawater: Effect of Pressure

The Zhang et al. model [Optics Express,17, 5698-5710 (2009)] for calculating light scattering by seawater doesnot account for pressure, which should, theoretically, affect molecular scattering. While negligible in nearsurface waters, the error associated with this approximation could be significant when backscattering is mea-sured directly in the deep ocean, by deep CTD casts or biogeochemical-Argo floats, for example. We updated theparameterization in the Zhang et al. model using (1) the Millard and Seaver equation for the refractive index ofseawater [Deep Sea Research Part A,37, 1909-1926 (1990)] and (2) the Feistel equation for Gibbs free energyfor seawater thermodynamics [Deep-Sea Research I,55, 1639-1671 (2008)]. As these equations include theeffect of pressure as well as salinity and temperature, our new parameterization allows us to investigate thepotential effect of pressure on scattering. Increasing pressure suppresses the random motion of molecules, re-ducing the fluctuations in both density and concentration, which in turn causes an overall decrease in lightscattering by seawater. For pure water and seawater with a salinity of 34 PSU, the decreases are approximately13% and 12%, respectively, with a 100-MPa (approximately the pressure of seawater at 10000 m) increase inpressure. Below the thermocline and/or halocline where temperature and salinity change slowly, the steadyincrease of pressure is the dominant factor affecting the light scattering by seawater. At depths where back-scattering is typically dominated by molecular scattering by seawater, particulate backscattering would beunderestimated if the effect of pressure on molecular scattering were not considered

A hybrid modal decomposition approach for higher-order modes in circular ducts

Before it is possible to determine the effect over a wide frequency range of different aperture devices on the sound field in a duct, the contribution from the individual higher-order modes must be established. Two approaches to decompose the sound field may be taken which are either to use a large number of microphone locations to reconstruct the sound field, or to use a hybrid method involving a reduced set of microphone locations and a model of the sound field in the system. Modelling the higher-order modes in a duct is itself a numerically intensive procedure if fully coupled calculations are required. It is possible to simplify the process for modelling the sound field by using uncoupled calculations for the higher order modes. Results are presented for such a hybrid approach, combining a limited number of microphone locations with an uncoupled model, to establish the sound field in a circular duct. Both point source and plane wave sources are considered and direct measurement of the sound field is compared to the reconstructed field for a normalised wave number range up to 7. Results show acceptable agreement between the hybrid approach and direct measurement with the greatest errors occurring around cut-on of the axially anti-symmetric modes. Thus, it is demonstrated that a hybrid approach may be applied to ducts with simple sources and that the approach can be used to deconstruct the in-duct sound field into the individual higher-order mode contribution

Sampling with Confidence: Using k-NN Confidence Measures in Active Learning

Active learning is a process through which classifiers can be built from collections of unlabelled examples through the cooperation of a human oracle who can label a small number of examples selected as most informative. Typically the most informative examples are selected through uncertainty sampling based on classification scores. However, previous work has shown that, contrary to expectations, there is not a direct relationship between classification scores and classification confidence. Fortunately, there exists a collection of particularly effective techniques for building measures of classification confidence from the similarity information generated by k-NN classifiers. This paper investigates using these confidence measures in a new active learning sampling selection strategy, and shows how the performance of this strategy is better than one based on uncertainty sampling using classification scores

EGAL: Exploration Guided Active Learning for TCBR

The task of building labelled case bases can be approached using active learning (AL), a process which facilitates the labelling of large collections of examples with minimal manual labelling effort. The main challenge in designing AL systems is the development of a selection strategy to choose the most informative examples to manually label. Typical selection strategies use exploitation techniques which attempt to refine uncertain areas of the decision space based on the output of a classifier. Other approaches tend to balance exploitation with exploration, selecting examples from dense and interesting regions of the domain space. In this paper we present a simple but effective exploration only selection strategy for AL in the textual domain. Our approach is inherently case-based, using only nearest-neighbour-based density and diversity measures. We show how its performance is comparable to the more computationally expensive exploitation-based approaches and that it offers the opportunity to be classifier independent

A Study of School Lunch Food Choice and Consumption among Elementary School Students

Background: School lunches that are part of the National School Lunch Program must meet specific nutrition requirements. It is unknown whether students eat school lunches in a balanced manner. This study examined which lunch food items children eat/toss, and consumption differences between sexes, and between students at a Non-Title 1 school where most students purchase school lunches and a Title 1 school where most students receive free or reduced-priced lunches. Methods: Students in both schools were observed unobtrusively for five consecutive days. Trained observers recorded the discarded food items on a checklist listing the menu items for the day. The final analyses (t- tests, z tests) included data from 2,826 student-meals. Results: Entrees similar to those offered in fast food restaurants (i.e., chicken nuggets, pizza, nachos, corndogs) were favored by most students. Fresh fruits and vegetables were not selected or consumed frequently. Boys consumed more food in the fast food, starches, and dessert categories, while girls consumed more soup, salads, and vegetables. Title 1 school students consumed more food in all categories except bread, pasta, and sandwiches than Non-Title 1 school students. Conclusions: Students’ preference of unhealthy lunch items may decrease the health benefits that school lunches attempt to provide

Supporting medical students to support peers : a qualitative interview study

Background Students may be the first to recognise and respond to psychological distress in other students. Peer support could overcome medical student reluctance to seek help despite their high rates of mental ill-health. Yet, despite the adoption of peer support programs, there is little evidence of impact on students. Peer support programs may assume that medical students accept and view peer support positively. We explored these assumptions by asking students about their experiences and views on peer support. Methods Qualitative semi-structured interviews exploring peer support experiences and views on peer support were conducted with ten medical students at two contrasting medical schools. Informed by a constructivist stance, interview transcripts underwent thematic analysis. Results Three groups of themes were identified: participants' experiences of peer support encounters, concerns about providing support, and views on students' roles in peer support. Participants readily recalled signs of peer distress. Encounters were ad hoc, informal, and occurred within relationships based on friendship or by being co-located in the same classes or placements. Concerns about initiating and offering support included lack of expertise, maintaining confidentiality, stigma from a mental health diagnosis, and unclear role boundaries, with implications for acceptance of student roles in peer support. Conclusions Our study emphasised the centrality of social relationships in enabling or discouraging peer support. Relationships developed during medical studies may anticipate the collegial relationships between medical professionals. Nevertheless, only some students are willing to undertake peer support roles. We suggest different strategies for promoting informal peer support that can be offered by any student, to those promoting formal support roles for selected students. Future research focusing on the impact for both the students who receive, and on the students who provide peer support is called for