40,049 research outputs found

    Quintic threefolds and Fano elevenfolds

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    The derived category of coherent sheaves on a general quintic threefold is a central object in mirror symmetry. We show that it can be embedded into the derived category of a certain Fano elevenfold. Our proof also generates related examples in different dimensions.Comment: V1: 12 pages. V2: added reference to work of Iliev and Manivel. V3: persistent sign error corrected. Other minor changes following referee's suggestions. To appear in Crell

    Rare earth elements in Andaman Island surface water : geochemical tracers for the monsoon

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    The Asian summer monsoon affects the lives of billions of people. With the aim of identifying geochemical tracers for the monsoon related freshwater input from the major rivers draining into the Bay of Bengal and Andaman Sea we have taken surface seawater samples from various locations up and down the Andaman Islands during 2011. Importantly, in some locations samples have been taken in March, July and November, covering most of a seasonal cycle and different monsoon phases. Samples were collected from the side of small wooden boats or while swimming and were filtered within a few hours at 0.45 or 0.22 microns using the vacuum produced by a water jet or a hand operated peristaltic pump. Filtered and unfiltered samples were acidified to < pH 2 and analysed for Y and the REEs with an automated online preconcentration ICP-MS technique [1]. The local input of REEs from streams and sediment rich areas such as mangrove environments is clearly identified by middle REE enrichments in the shale normalised patterns of some samples. These middle REE bulges accompany large increases in dissolved REE concentrations at some locations, especially for the July samples obtained during the peak monsoon season with frequent storms. Y/Ho fractionation aslo occurs during the local input of dissolved REEs with affected samples having lower Y/Ho ratios. Conversly, some samples, in particular those taken after heavy rainfall in March, show strong REE scavenging accompanied by the prefferential removal of dissolved light REEs and higher Y/Ho ratios. The time series at a location away from local input sources shows remarkably similar REE patterns and concentraions in March and July. Then in October-November, following the peak in monsoon river discharge, the dissolved REE concentration increases by almost a factor of 2. The notable exception to this seasonal pattern is the Ce anomally which is around 0.3 in March and November but 0.6 in July, implying less oxidative removal of Ce(IV) during the peak summer monsoon rains. With the exception of elevated dissolved Ce concentrations, the North Pacific Deep Water normalised REE patterns are similar to those reported for offshore samples from the Bay of Bengal and Andaman Sea [2]. These seawater normalised patterns are distinctive having a middle REE enriched arc with similar light and heavy REE values suggesting the input from large rivers in the region is traceable using seawater REE chemistry. [1] Hathorne et al. (2012), Online preconcentration ICP-MS analysis of rare earth elements in seawater, Geochem. Geophys. Geosyst., 13, Q01020, doi:10.1029/2011GC003907. [2] Amakawa et al. (2000), Geochim. Cosmochim. Acta 64, 1715-1727

    Classroom contexts: Connections between class size and within class grouping

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    Background. There has been a vigorous debate for many years about the educational effects of class size differences, but even if differences have an impact on pupils' academic progress this still leaves unanswered important questions about what mediates the effect. Aims. This paper is informed by a classroom contextual perspective, and examines associations between class size and within class groupings (in terms of size and number of groups, adult presence in groups, and type of interaction between grouping members). Age differences in these relationships are also explored. Samples. The quantitative study is based on analysis of 3157 groupings, from 672 Reception, Year 2 and Year 5 classes in 331 schools. The qualitative study was based on 12 classes in 8 case study schools, and questionnaire responses completed by over 100 class teachers. Methods. Links between size of class and within class groupings were examined on the basis of a 'grouping mapping survey', in which teachers at a given time in the school day provided information on group size and number, adult presence, and type of interaction between pupils, and complementary qualitative analyses of data from teacher-completed questionnaires, and interviews. Results. The number of groups in a class increased with the size of the class. Over all three year groups, small classes had on average just over three groups, while large classes approached six groups. The size of groups in the class decreased with size of class. In class sizes over 25, pupils were more likely to be in a large group of 7-10, while in classes under 25 they were more likely to be in whole class groupings. Qualitative analyses showed that teachers felt that groups of 7-10 pupils had negative educational effects, for example, in terms of the quality and quantity of teaching and children's concentration and contribution in groups. Conclusions. Results suggest that the effects of class size can be best seen as through the size and number of groups, which will then have implications for learning experiences. So while debate about size of class has often been in terms of reduced size of class resulting in pupil academic gains, it is also important educationally to consider within class grouping size and number, and their effects.link_to_subscribed_fulltex

    Toward a social pedagogy of classroom group work

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    In any classroom, pupils will be drawn together for many purposes and we can refer to such within classroom contexts as 'groupings'. The teacher often creates these, and the way that they are set up, and how they are used for particular learning purposes. If the relationships between grouping size, interaction type and learning tasks in groups are planned strategically then learning experiences will be more effective. However, research suggests that the relationships between these elements are often unplanned and the 'social pedagogic' potential of classroom learning is therefore unrealised. In this paper we explore the notion of social pedagogy in relation to group work. It is argued that research and theory relevant to group work in classrooms is limited, and that a new approach, sensitive to group work under everyday classroom conditions is required. This paper identifies key features of a social pedagogy of classroom group work, which can inform effective group work in classrooms. It also describes the background to a current large scale UK project which has been set up to design with teachers a programme of high quality group work in classrooms at both primary and secondary phases. © 2003 Elsevier Ltd. All rights reserved.link_to_subscribed_fulltex

    Population connectivity shifts at high frequency within an open-coast marine protected area network.

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    A complete understanding of population connectivity via larval dispersal is of great value to the effective design and management of marine protected areas (MPA). However empirical estimates of larval dispersal distance, self-recruitment, and within season variability of population connectivity patterns and their influence on metapopulation structure remain rare. We used high-resolution otolith microchemistry data from the temperate reef fish Hypsypops rubicundus to explore biweekly, seasonal, and annual connectivity patterns in an open-coast MPA network. The three MPAs, spanning 46 km along the southern California coastline were connected by larval dispersal, but the magnitude and direction of connections reversed between 2008 and 2009. Self-recruitment, i.e. spawning, dispersal, and settlement to the same location, was observed at two locations, one of which is a MPA. Self-recruitment to this MPA ranged from 50-84%; within the entire 60 km study region, self-recruitment accounted for 45% of all individuals settling to study reefs. On biweekly time scales we observed directional variability in alongshore current data and larval dispersal trajectories; if viewed in isolation these data suggest the system behaves as a source-sink metapopulation. However aggregate biweekly data over two years reveal a reef network in which H. rubicundus behaves more like a well-mixed metapopulation. As one of the few empirical studies of population connectivity within a temperate open coast reef network, this work can inform the MPA design process, implementation of ecosystem based management plans, and facilitate conservation decisions

    Comment on "A non-interacting low-mass black hole -- giant star binary system"

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    Thompson et al. (Reports, 1 November 2019, p. 637, Science) interpreted the unseen companion of the red giant star 2MASS J05215658+4359220 as most likely a black hole. We argue that if the red giant is about one solar mass, its companion can be a close binary consisting of two main-sequence stars. This would explain why no X-ray emission is detected from the system.Comment: 3 pages, Author version of Technical Comment published in Science on 8 May, 202

    An investigation into linearity with cumulative emissions of the climate and carbon cycle response in HadCM3LC

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    We investigate the extent to which global mean temperature, precipitation, and the carbon cycle are constrained by cumulative carbon emissions throughout four experiments with a fully coupled climate-carbon cycle model. The two paired experiments adopt contrasting, idealised approaches to climate change mitigation at different action points this century, with total emissions exceeding two trillion tonnes of carbon in the later pair. Their initially diverging cumulative emissions trajectories cross after several decades, before diverging again. We find that their global mean temperatures are, to first order, linear with cumulative emissions, though regional differences in temperature of up to 1.5K exist when cumulative emissions of each pair coincide. Interestingly, although the oceanic precipitation response scales with cumulative emissions, the global precipitation response does not, due to a decrease in precipitation over land above cumulative emissions of around one trillion tonnes of carbon (TtC). Most carbon fluxes and stores are less well constrained by cumulative emissions as they reach two trillion tonnes. The opposing mitigation approaches have different consequences for the Amazon rainforest, which affects the linearity with which the carbon cycle responds to cumulative emissions. Averaged over the two fixed-emissions experiments, the transient response to cumulative carbon emissions (TCRE) is 1.95 K TtC-1, at the upper end of the IPCC’s range of 0.8-2.5 K TtC-1
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