1,385 research outputs found

    Cusps of Hilbert modular varieties

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    Motivated by a question of Hirzebruch on the possible topological types of cusp cross-sections of Hilbert modular varieties, we give a necessary and sufficient condition for a manifold M to be diffeomorphic to a cusp cross-section of a Hilbert modular variety. Specialized to Hilbert modular surfaces, this proves that every Sol 3-manifold is diffeomorphic to a cusp cross-section of a (generalized) Hilbert modular surface. We also deduce an obstruction to geometric bounding in this setting. Consequently, there exist Sol 3-manifolds that cannot arise as a cusp cross-section of a 1-cusped nonsingular Hilbert modular surface.Comment: To appear in Mathematical Proceedings Cambridge Philosophical Societ

    Let's reflect on processes : task uncertainty as a moderator for feedback effectiveness

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    Traditionally, when introducing feedback interventions, the focus of performance indicators used as the basis for feedback has mainly been on the final results of tasks. This stemmed from the general contention that providing employees with information about their performance on the final results of their work unconditionally increases their performance. Only over the last few decades have researchers come to realize that findings regarding feedback effectiveness have not been consistent. More recent research has already suggested that several characteristics of feedback and task might act as significant moderators. However, until now, these moderating conditions remained poorly understood. The research reported in this dissertation aimed to contribute to the understanding of feedback effectiveness and examined the until now underexplored combined moderating effect of task uncertainty, type of feedback (outcome versus process feedback) and feedback reflection on feedback effectiveness. The main research question, ‘Dependent on the level of task uncertainty, what type offeedback should employees be provided with for feedback to be effective?’, was addressed in three separate, yet closely related studies. In all studies, the well validated ProMES method (Productivity Measurement and Enhancement System) was used for the development and provision of performance feedback. In the first study (Chapter 2), a meta-analysis, the combined moderating effect of task uncertainty, type of feedback, and feedback reflection on the effectiveness of ProMES feedback was examined using performance data from 83 field studies from a wide variety of organizational settings. Results indicated that when task uncertainty is low, employees higher on reflection on feedback outperform employees lower on reflection on feedback, irrespective of the type of feedback they receive. However, when task uncertainty is high, employees higher on reflection on feedback outperform employees lower on reflection on feedback when the proportion of process feedback is higher. Moreover, the reverse is true when the proportion of process feedback is lower. Thus, this study showed through meta-analysis over a large variety of tasks, teams, organizations, and industries that the effectiveness of feedback on performance is not at all straightforward. Task uncertainty, type of feedback, and reflection on feedback appear to be important moderating conditions for feedback effectiveness, where some combinations of these variables can lead to very large positive effects and others can actually lead to negative effects on performance. In the second study (Chapter 3), a quasi-field experiment, it was examined whether task uncertainty influences the type of performance indicators participatively developed by practitioners from the field. For this purpose, a task uncertainty framework was defined. Then, 50 care providing employees divided over 8 medical rehabilitation teams varying on task uncertainty participated in the development of performance feedback systems using the ProMES method. Results indicated that teams higher on task uncertainty develop relatively more process indicators (compared to outcome indicators) than teams lower on task uncertainty. Moreover, in line with the task uncertainty framework, process indicators developed by teams higher on task uncertainty are more of a problem solving nature, whereas process indicators developed by teams lower on task uncertainty are more of a procedural nature. Thus, this study showed through quasi-field research in health care that the level of uncertainty employees are dealing with during care provision determines which types of indicators are regarded as helpful with the successful fulfillment of their tasks. In the third study (Chapter 4), a quasi-field experiment, it was examined whether task uncertainty, type of feedback, and feedback reflection have a moderating effect on performance with the sequential introduction of outcome and process feedback. For this purpose, 107 care providing employees, belonging to 4 medical rehabilitation teams varying on task uncertainty, periodically received performance feedback through ProMES feedback reports, which were discussed in feedback meetings. Results indicated that a three-way interaction exists between the level of task uncertainty, the type of feedback, and the time spent on reflection on feedback during feedback meetings, such that only with higher levels of task uncertainty and higher levels of reflection process feedback results in higher performance than outcome feedback. In addition, longitudinal questionnaire data from arepeated measures design with three time waves were used in this study to examine the combined effect of task uncertainty and type of feedback on factors enabling the development and use of task knowledge, such as coping with task uncertainty, task information sharing, role clarity, and empowerment. The results indicated that with higher levels of task uncertainty, only the introduction of process feedback (compared to outcome feedback) has a positive effect on these supposedly performance-enhancing factors. Thus, this study showed through a quasi-field experiment in health care that the effectiveness of feedback is dependent on the level of task uncertainty, the type of feedback, and the level of reflection on feedback. Through reflection on process feedback, employees dealing with higher levels of task uncertainty are presented with the opportunity to develop and use appropriate task knowledge. In conclusion, the research presented in this dissertation confirmed through meta analysis and empirical field research that the effects of feedback are not always the same. Instead, moderating conditions such as task uncertainty, type of feedback, and reflection on feedback play important roles in the effects of feedback, both on performance and on underlying psychological factors such as coping with task uncertainty, task information sharing, role clarity, and empowerment. Ignoring these findings when designing and implementing performance feedback systems could be harmful for organizations

    New electron source concept for single-shot sub-100 fs electron diffraction in the 100 keV range

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    We present a method for producing sub-100 fs electron bunches that are suitable for single-shot ultrafast electron diffraction experiments in the 100 keV energy range. A combination of analytical results and state-of-the-art numerical simulations show that it is possible to create 100 keV, 0.1 pC, 20 fs electron bunches with a spotsize smaller than 500 micron and a transverse coherence length of 3 nm, using established technologies in a table-top set-up. The system operates in the space-charge dominated regime to produce energy-correlated bunches that are recompressed by established radio-frequency techniques. With this approach we overcome the Coulomb expansion of the bunch, providing an entirely new ultrafast electron diffraction source concept

    Compression of sub-relativistic space-charge-dominated electron bunches for single-shot femtosecond electron diffraction

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    We demonstrate compression of 95 keV, space-charge-dominated electron bunches to sub-100 fs durations. These bunches have sufficient charge (200 fC) and are of sufficient quality to capture a diffraction pattern with a single shot, which we demonstrate by a diffraction experiment on a polycrystalline gold foil. Compression is realized by means of velocity bunching as a result of a velocity chirp, induced by the oscillatory longitudinal electric field of a 3 GHz radio-frequency cavity. The arrival time jitter is measured to be 80 fs

    Curves of every genus with many points, I: Abelian and toric families

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    Let N_q(g) denote the maximal number of F_q-rational points on any curve of genus g over the finite field F_q. Ihara (for square q) and Serre (for general q) proved that limsup_{g-->infinity} N_q(g)/g > 0 for any fixed q. In their proofs they constructed curves with many points in infinitely many genera; however, their sequences of genera are somewhat sparse. In this paper, we prove that lim_{g-->infinity} N_q(g) = infinity. More precisely, we use abelian covers of P^1 to prove that liminf_{g-->infinity} N_q(g)/(g/log g) > 0, and we use curves on toric surfaces to prove that liminf_{g-->infty} N_q(g)/g^{1/3} > 0; we also show that these results are the best possible that can be proved with these families of curves.Comment: LaTeX, 20 page

    Wolfram Syndrome protein, Miner1, regulates sulphydryl redox status, the unfolded protein response, and Ca2+ homeostasis.

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    Miner1 is a redox-active 2Fe2S cluster protein. Mutations in Miner1 result in Wolfram Syndrome, a metabolic disease associated with diabetes, blindness, deafness, and a shortened lifespan. Embryonic fibroblasts from Miner1(-/-) mice displayed ER stress and showed hallmarks of the unfolded protein response. In addition, loss of Miner1 caused a depletion of ER Ca(2+) stores, a dramatic increase in mitochondrial Ca(2+) load, increased reactive oxygen and nitrogen species, an increase in the GSSG/GSH and NAD(+)/NADH ratios, and an increase in the ADP/ATP ratio consistent with enhanced ATP utilization. Furthermore, mitochondria in fibroblasts lacking Miner1 displayed ultrastructural alterations, such as increased cristae density and punctate morphology, and an increase in O2 consumption. Treatment with the sulphydryl anti-oxidant N-acetylcysteine reversed the abnormalities in the Miner1 deficient cells, suggesting that sulphydryl reducing agents should be explored as a treatment for this rare genetic disease

    The effect of area and isolation on insular dwarf proboscideans

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    Aim We investigated the hypothesis that insular body size of fossil elephants is directly related to isolation and surface area of the focal islands. Location Palaeo-islands worldwide. Methods We assembled data on the geographical characteristics (area and isolation) of islands and body size evolution of palaeo-insular species for 22 insular species of fossil elephants across 17 islands. Results Our results support the generality of the island rule in the sense that all but one of the elephants experienced dwarfism on islands. The smallest islands generally harbour the smallest elephants. We found no support for the hypothesis that body size of elephants declines with island isolation. Body size is weakly and positively correlated with island area for proboscideans as a whole, but more strongly correlated for Stegodontidae when considered separately. Average body size decrease is much higher when competitors are present. Main conclusions Body size in insular elephants is not significantly correlated with the isolation of an island. Surface area, however, is a significant predictor of body size. The correlation is positive but relatively weak; c. 23% of the variation is explained by surface area. Body size variation seems most strongly influenced by ecological interactions with competitors, possibly followed by time in isolation. Elephants exhibited far more extreme cases of dwarfism than extant insular mammals, which is consistent with the substantially more extended period of deep geological time that the selective pressures could act on these insular populations

    The class of the locus of intermediate Jacobians of cubic threefolds

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    We study the locus of intermediate Jacobians of cubic threefolds within the moduli space of complex principally polarized abelian fivefolds, and its generalization to arbitrary genus - the locus of abelian varieties with a singular odd two-torsion point on the theta divisor. Assuming that this locus has expected codimension (which we show to be true for genus up to 5), we compute the class of this locus, and of is closure in the perfect cone toroidal compactification, in the Chow, homology, and the tautological ring. We work out the cases of genus up to 5 in detail, obtaining explicit expressions for the classes of the closures of the locus of products of an elliptic curve and a hyperelliptic genus 3 curve, in moduli of principally polarized abelian fourfolds, and of the locus of intermediate Jacobians in genus 5. In the course of our computation we also deal with various intersections of boundary divisors of a level toroidal compactification, which is of independent interest in understanding the cohomology and Chow rings of the moduli spaces.Comment: v2: new section 9 on the geometry of the boundary of the locus of intermediate Jacobians of cubic threefolds. Final version to appear in Invent. Mat
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