941 research outputs found

    Distribution and Abundance of Molluscs in a Fresh Water Environment

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    The molluscs of o northwestern Minnesota lake were sampled using transects, a sampling frame, and SCUBA. The species sampled were: Amnicola limosa, Valvata tricarinata, Gyraulus parvus, Physa cf. P. gyrina, Helisoma anceps, H. campanulata, Promenetus exacuous, Ferrissia parallela, Anodonta marginata, Lampsilis siliquoidea, and Sphaerium cf. S. striatum. The unionid clams, the adult Helisoma spp. and the Physa adults were associated with the absence of aquatic vegetation. Distinct associations of snail species were found with each plant association: G. parvus and V. tricarinata with the deep water Nitella opaca association, A. limosa and V. tricarinata with the mid-depth mixed macrophyte association, and H. anceps and Physa adults with the shallow water, rocky bottom macrophyte association. Two estimates of snail abundance were made for each depth. Neither proved very satisfactory, but they did indicate that snail abundance was related to the type and abundance of aquatic plants

    Density mapping with weak lensing and phase information

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    The available probes of the large scale structure in the Universe have distinct properties: galaxies are a high resolution but biased tracer of mass, while weak lensing avoids such biases but, due to low signal-to-noise ratio, has poor resolution. We investigate reconstructing the projected density field using the complementarity of weak lensing and galaxy positions. We propose a maximum-probability reconstruction of the 2D lensing convergence with a likelihood term for shear data and a prior on the Fourier phases constructed from the galaxy positions. By considering only the phases of the galaxy field, we evade the unknown value of the bias and allow it to be calibrated by lensing on a mode-by-mode basis. By applying this method to a realistic simulated galaxy shear catalogue, we find that a weak prior on phases provides a good quality reconstruction down to scales beyond l=1000, far into the noise domain of the lensing signal alone.Comment: 11 pages, 9 figures, published in MNRA

    A Test of Risk Vulnerability in the Wider Population

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    Panel data from the German SOEP is used to test for risk vulnerability (RV) in the wider population. Two different survey responses are analysed: the response to the question about willingness-to-take risk in general and the chosen investment in a hypothetical lottery. A convenient indicator of background risk is the VDAX index, an established measure of volatility in the German stock market. This is used as an explanatory variable in conjunction with HDAX, the stock market index, which proxies wealth. The impacts of these measures on risk attitude are identifiable by exploiting the time dimension of the panel and matching survey months with corresponding observations from these time-varying factors. Both of the survey responses allow us to test for decreasing absolute risk aversion (DARA); in one case, we find strong evidence of DARA, while in the other, we do not. Both survey responses also allow us to test for RV, and in both cases we find strong evidence. In the case of the hypothetical lottery response, we are also able to estimate a “coefficient of risk vulnerability” (CRV). This is defined as the absolute amount by which absolute risk aversion rises in response to a doubling of background risk. We estimate CRV to be between 1.03 and 1.27
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