27,557 research outputs found

    Subsolar-meridian mean annual distributions for Martian troposphere below 50 kilometers

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    Subsolar-meridian mean annual distributions for Martian troposphere below 50 k

    High performance sapphire windows

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    High-quality, wide-aperture optical access is usually required for the advanced laser diagnostics that can now make a wide variety of non-intrusive measurements of combustion processes. Specially processed and mounted sapphire windows are proposed to provide this optical access to extreme environment. Through surface treatments and proper thermal stress design, single crystal sapphire can be a mechanically equivalent replacement for high strength steel. A prototype sapphire window and mounting system have been developed in a successful NASA SBIR Phase 1 project. A large and reliable increase in sapphire design strength (as much as 10x) has been achieved, and the initial specifications necessary for these gains have been defined. Failure testing of small windows has conclusively demonstrated the increased sapphire strength, indicating that a nearly flawless surface polish is the primary cause of strengthening, while an unusual mounting arrangement also significantly contributes to a larger effective strength. Phase 2 work will complete specification and demonstration of these windows, and will fabricate a set for use at NASA. The enhanced capabilities of these high performance sapphire windows will lead to many diagnostic capabilities not previously possible, as well as new applications for sapphire

    Cosmological production of H_2 before the formation of the first galaxies

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    Previous calculations of the pregalactic chemistry have found that a small amount of H_2, x[H_2]=n[H_2]/n[H] = 2.6e-6, is produced catalytically through the H^-, H_2^+, and HeH^+ mechanisms. We revisit this standard calculation taking into account the effects of the nonthermal radiation background produced by cosmic hydrogen recombination, which is particularly effective at destroying H^- via photodetachment. We also take into consideration the non-equilibrium level populations of H_2^+, which occur since transitions among the rotational-vibrational levels are slow compared to photodissociation. The new calculation predicts a final H_2 abundance of x[H_2] = 6e-7 for the standard cosmology. This production is due almost entirely to the H^- mechanism, with ~1 per cent coming from HeH^+ and ~0.004 per cent from H_2^+. We evaluate the heating of the diffuse pregalactic gas from the chemical reactions that produce H_2 and from rotational transitions in H_2, and find them to be negligible.Comment: 13 pages, 5 figures, MNRAS submitte

    A technique for solving certain Wiener-Hopf type boundary value problems Technical report no. 9

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    Technique for solving Weiner-Hopf type boundary value problem

    An alternative approach to the solution of a class of Wiener-Hopf and related problems Technical report no. 8

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    Alternative method to Weiner-Hopf approach for solving radiation and diffraction problem

    Elephant cognition in primate perspective

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    On many of the staple measures of comparative psychology, elephants show no obvious differences from other mammals, such as primates: discrimination learning, memory, spontaneous tool use, etc. However, a range of more naturalistic measures have recently suggested that elephant cognition may be rather different. Wild elephants sub-categorize humans into groups, independently making this classification on the basis of scent or colour. In number discrimination, elephants show no effects of absolute magnitude or relative size disparity in making number judgements. In the social realm, elephants show empathy into the problems faced by others, and give hints of special abilities in cooperation, vocal imitation and perhaps teaching. Field data suggest that the elephant’s vaunted reputation for memory may have a factual basis, in two ways. Elephants’ ability to remember large-scale space over long periods suggests good cognitive mapping skills. Elephants’ skill in keeping track of the current locations of many family members implies that working memory may be unusually developed, consistent with the laboratory finding that their quantity judgements do not show the usual magnitude effects.Publisher PDFPeer reviewe

    A note on commuting graphs for symmetric groups

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    The commuting graph C(G;X) , where G is a group and X a subset of G, has X as its vertex set with two distinct elements of X joined by an edge when they commute in G. Here the diameter and disc structure of C(G;X) is investigated when G is the symmetric group and X a conjugacy class of G

    Strengths of sulfur-basalt concretes

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    Sulfur used in bonding high strength basalt aggregates to form sulfur-basalt concrete

    Transitions to Entrepreneurship and Industry-Specific Barriers

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    Drivers of entrepreneurial entry are investigated in this study by examining how entry into small-business ownership is shaped by industry-specific constraints. The human- and financial-capital endowments of potential entrepreneurs entering firms in various industries are shown to differ profoundly, depending on the type of venture entered. The educational credentials of highly educated potential entrepreneurs, in particular, predict avoidance of small-firm ownership in some industries as well as attraction to others. Recognizing that individuals choose an industry sector jointly with their decision to enter entrepreneurship, we find that the conventional practice of conflating different industry types in empirical analyses of transitions to entrepreneurship generates misleading findings about the determinants of entrepreneurship.entrepreneurship, self-employment, capital constraints, transitions, entry barriers, business start-ups
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