Sinks in Acyclic Orientations of Graphs

Greene and Zaslavsky proved that the number of acyclic orientations of a graph with a unique sink is, up to sign, the linear coefficient of the chromatic polynomial. We give three new proofs of this result using pure induction, noncommutative symmetric functions, and an algorithmic bijection.Comment: 17 pages, 1 figur

Science for Place-based Socioecological Management: Lessons from the Maya Forest (Chiapas and Petén)

The role humans should play in conservation is a pervasive issue of debate in environmental thinking. Two long-established poles of this debate can be identified on a preservation-sustainable use continuum. At one extreme are use bans and natural science-based, top-down management for preservation. At the other extreme is community-based, multidisciplinary management for sustainable resource use and livelihoods. In this paper, we discuss and illustrate how these two strategies have competed and conflicted in conservation initiatives in the Maya forest (MF) of the Middle Usumacinta River watershed (Guatemala and Mexico). We further argue that both extremes have produced unconvincing results in terms of the region’s sustainability. An alternative consists of sustainability initiatives based on place-based and integrated-knowledge approaches. These approaches imply a flexible combination of disciplines and types of knowledge in the context of nature-human interactions occurring in a place. They can be operationalized within the framework of sustainability science in three steps: 1) characterize the contextual circumstances that are most relevant for sustainability in a place; 2) identify the disciplines and knowledge(s) that need to be combined to appropriately address these contextual circumstances; and 3) decide how these disciplines and knowledge can be effectively combined and integrated. Epistemological flexibility in the design of analytic and implementation frameworks is key. Place-based and integrative-knowledge approaches strive to deal with local context and complexity, including that of human individuals and cultures. The success of any sustainability initiative will ultimately depend on its structural coupling with the context in which it is applied

Multi-wavelength holography with a single spatial light modulator for ultracold atom experiments

The authors acknowledge funding from the Leverhulme Trust Research Project Grant RPG-2013-074 and from the EPSRC grant GR/T08272/01.We demonstrate a method to independently and arbitrarily tailor the spatial profile of light of multiple wavelengths and we show possible applications to ultracold atoms experiments. A single spatial light modulator is programmed to create a pattern containing multiple spatially separated structures in the Fourier plane when illuminated with a single wavelength. When the modulator is illuminated with overlapped laser beams of different wavelengths, the position of the structures is wavelength-dependent. Hence, by designing their separations appropriately, a desired overlap of different structures at different wavelengths is obtained. We employ regional phase calculation algorithms and demonstrate several possible experimental scenarios by generating light patterns with 670 nm, 780 nm and 1064 nm laser light which are accurate to the level of a few percent. This technique is easily integrated into cold atom experiments, requiring little optical access.PostprintPeer reviewe

The use of chronosequences in studies of ecological succession and soil development

1. Chronosequences and associated space-for-time substitutions are an important and often necessary tool for studying temporal dynamics of plant communities and soil development across multiple time-scales. However, they are often used inappropriately, leading to false conclusions about ecological patterns and processes, which has prompted recent strong criticism of the approach. Here, we evaluate when chronosequences may or may not be appropriate for studying community and ecosystem development. 2. Chronosequences are appropriate to study plant succession at decadal to millennial time-scales when there is evidence that sites of different ages are following the same trajectory. They can also be reliably used to study aspects of soil development that occur between temporally linked sites over time-scales of centuries to millennia, sometimes independently of their application to shorter-term plant and soil biological communities. 3. Some characteristics of changing plant and soil biological communities (e.g. species richness, plant cover, vegetation structure, soil organic matter accumulation) are more likely to be related in a predictable and temporally linear manner than are other characteristics (e.g. species composition and abundance) and are therefore more reliably studied using a chronosequence approach. 4. Chronosequences are most appropriate for studying communities that are following convergent successional trajectories and have low biodiversity, rapid species turnover and low frequency and severity of disturbance. Chronosequences are least suitable for studying successional trajectories that are divergent, species-rich, highly disturbed or arrested in time because then there are often major difficulties in determining temporal linkages between stages. 5. Synthesis. We conclude that, when successional trajectories exceed the life span of investigators and the experimental and observational studies that they perform, temporal change can be successfully explored through the judicious use of chronosequences

Lessons to be Learned from Natural Control of HIV – Future Directions, Therapeutic, and Preventive Implications

Accumulating data generated from persons who naturally control HIV without the need for antiretroviral treatment has led to significant insights into the possible mechanisms of durable control of AIDS virus infection. At the center of this control is the HIV-specific CD8 T cell response, and the basis for this CD8-mediated control is gradually being revealed. Genome wide association studies coupled with HLA sequence data implicate the nature of the HLA-viral peptide interaction as the major genetic factor modulating durable control of HIV, but host genetic factors account for only around 20% of the variability in control. Other factors including specific functional characteristics of the TCR clonotypes generated in vivo, targeting of vulnerable regions of the virus that lead to fitness impairing mutations, immune exhaustion, and host restriction factors that limit HIV replication all have been shown to additionally contribute to control. Moreover, emerging data indicate that the CD8+ T cell response may be critical for attempts to purge virus infected cells following activation of the latent reservoir, and thus lessons learned from elite controllers (ECs) are likely to impact the eradication agenda. On-going efforts are also needed to understand and address the role of immune activation in disease progression, as it becomes increasingly clear that durable immune control in ECs comes at a cost. Taken together, the research achievements in the attempt to unlock the mechanisms behind natural control of HIV will continue to be an important source of insights and ideas in the continuous search after an effective HIV vaccine, and for the attempts to achieve a sterilizing or functional cure in HIV positive patients with progressive infection

Evaluating relative performances in disabled sports competitions

Handicapping systems play important roles in many sports. This paper focuses on disabled sports competitions, which primarily aim to encourage participation. Nevertheless, prizes are awarded for relative performances and should be allocated fairly. We review handicapping procedures specifically for disabled Alpine skiing and identify imperfections with this system, particularly how historical and new results are combined to determine scaling factors. Significant problems also arise due to different class sizes and variations in courses and conditions. We investigate the difficulties in establishing a suitable handicapping system and propose modifications to the existing procedures that improve their fairness. We develop simple mathematical models and statistical analyses that might be suitable, based on results data relating to several years of competition, and we evaluate the performance of our proposed method in comparison with that of the existing approac

Term Clustering of Syntactic Phrases

Term clustering and syntactic phrase formation are methods for transforming natural language text. Both have had only mixed success as strategies for improving the quality of text representations for document retrieval. Since the strengths of these methods are complementary, we have explored combining them to produce superior representations. In this paper we discuss our implementation of a syntactic phrase generator, as well as our preliminary experiments with producing phrase clusters. These experiments show small improvements in retrieval effectiveness resulting from the use of phrase clusters, but it is clear that corpora much larger than standard information retrieval test collections will be required to thoroughly evaluate the use of this technique

Tree and string analysis of a Copala Trique sentence

From the introduction: This paper is simply a presentation of two analyses of the same Copala Trique sentence. The first analysis is done by David Thomas and represents a string analysis, with a distinction between deep structure and surface structure. The second analysis is done by Bruce Hollenbach and represents a tree analysis done from the point of view of generative semantics. We hope that this presentation may be useful to those who might be interested in comparing and contrasting these two analytical techniques