38,151 research outputs found

    An update on the middle levels problem

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    The middle levels problem is to find a Hamilton cycle in the middle levels, M_{2k+1}, of the Hasse diagram of B_{2k+1} (the partially ordered set of subsets of a 2k+1-element set ordered by inclusion). Previously, the best result was that M_{2k+1} is Hamiltonian for all positive k through k=15. In this note we announce that M_{33} and M_{35} have Hamilton cycles. The result was achieved by an algorithmic improvement that made it possible to find a Hamilton path in a reduced graph of complementary necklace pairs having 129,644,790 vertices, using a 64-bit personal computer.Comment: 11 pages, 5 figure

    Effects of Early-Adolescent, Mid-Adolescent, or Adult Stress on Morphine Conditioned Place Preference

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    In light of previous work demonstrating that stress can increase subjective drug reward in adult rats, the present study investigated the influence of stress on morphine conditioned place preference (CPP) in early-adolescent, mid-adolescent, and adult male Sprague-Dawley rats. Subjects in each age group were assigned to either a no stress condition or a stress condition in which they were exposed to an unpredictable eight-day schedule of elevated platform and synthetic fox odor stressors. Place conditioning then evaluated subjective morphine reward in all animals. Using a biased procedure, subjects were assigned to receive morphine on the initially non-preferred side of the apparatus and saline on the initially preferred side as identified at pretest. After eight days of conditioning in which drug presentation alternated by day (i.e., morphine one day and saline the next), a post-test was conducted identical to pre-test. Results comparing pre- and post-test time on the non-preferred side indicated no difference between stress conditions or age groups, though place preference was observed in all animals. Activity, scored as midline crosses during conditioning trials, revealed expected habituation to the motor suppressing effects of morphine. Although no effects of stress or age were observed on measures of drug conditioning, the findings suggest that duration between the end of stress exposure and the start of conditioning may have weakened any effect of age-dependent stress on morphine CPP

    A normalised seawater strontium isotope curve: possible implications for Neoproterozoic-Cambrian weathering rates and the further oxygenation of the Earth

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    The strontium isotope composition of seawater is strongly influenced on geological time scales by changes in the rates of continental weathering relative to ocean crust alteration. However, the potential of the seawater 87Sr/86Sr curve to trace globally integrated chemical weathering rates has not been fully realised because ocean 87Sr/86Sr is also influenced by the isotopic evolution of Sr sources to the ocean. A preliminary attempt is made here to normalise the seawater 87Sr/86Sr curve to plausible trends in the 87Sr/86Sr ratios of the three major Sr sources: carbonate dissolution, silicate weathering and submarine hydrothermal exchange. The normalised curve highlights the Neoproterozoic-Phanerozoic transition as a period of exceptionally high continental influence, indicating that this interval was characterised by a transient increase in global weathering rates and/or by the weathering of unusually radiogenic crustal rocks. Close correlation between the normalised 87Sr/86Sr curve, a published seawater δ34S curve and atmospheric pCO2 models is used here to argue that elevated chemical weathering rates were a major contributing factor to the steep rise in seawater 87Sr/86Sr from 650 Ma to 500 Ma. Elevated weathering rates during the Neoproterozoic-Cambrian interval led to increased nutrient availability, organic burial and to the further oxygenation of Earth's surface environment. Use of normalised seawater 87Sr/86Sr curves will, it is hoped, help to improve future geochemical models of Earth System dynamics

    ‘My work is bleeding’: exploring students’ emotional responses to first-year assignment feedback

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    This paper explores the emotional responses that assignment feedback can provoke in first-year undergraduates. The literature on the link between emotions and learning is well established, but surprisingly research on the relationship between emotions and feedback is still relatively scarce. This article aims to make an additional contribution to this emerging field. Semi-structured interviews with 24 first-year undergraduate students from the Humanities and Social Sciences department in a post-1992 institution were conducted. The interview narratives identified how the emotional impact of feedback was related to: prior experiences of education, the significance participants attached to the feedback received on their first assignment and how their interpretations of feedback comments were linked to beliefs about themselves as learners. The implications of these experiences on student ‘belonging’ and learning are discussed. The underlying themes that emerged from the findings are the polarised emotions of anxiety and confidence. Based on the findings, the paper concludes by making recommendations for reconceptualising feedback on first-year assignments. It suggests that a holistic assessment approach, which incorporates timely feedback indicating if students are ‘on the right lines’ with low-stakes assignments, is a practice that may both reduce anxiety and increase confidence to support students

    Refracting the Male Gaze: Mary Cassatt’s Ocularcentric Message of Female Agency

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    Through analysis of ocularcentric (vision-privileged) messages in Mary Cassatt’s Reading Le Figaro (1878), I argue the portrait represents a strong early-career interest in countering gender hegemony that is largely unmatched in the artist’s later work. Reading the painting as a reaction to decreased autonomy following a consolidation of family living spaces, the present paper satisfies a dearth in Cassatt scholarship by addressing a yet-undiscussed motif of vision in Cassatt’s painting: the mirror. In a complete subversion of classic vanitas paintings, Cassatt alters the woman-and-mirror trope to emphasize her subject’s desire for an identity of female mind over female body. Defying European culture’s hyperspecularization (concentration on display value), which converted women into objects of male desire, Reading Le Figaro challenges preconceived schemas of gender and offers autonomy, self-definition, and active female sight as alternatives to the gaze-controlling subjugation of women

    X-ray source uses interchangeable target anodes to vary X-ray wavelength

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    Compact laboratory X ray tube generates X rays of various wavelengths by using interchangeable target anodes. The wavelength of the X rays depends on the metal from which the anode is made