2,813 research outputs found

    Simple extractors via constructions of cryptographic pseudo-random generators

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    Trevisan has shown that constructions of pseudo-random generators from hard functions (the Nisan-Wigderson approach) also produce extractors. We show that constructions of pseudo-random generators from one-way permutations (the Blum-Micali-Yao approach) can be used for building extractors as well. Using this new technique we build extractors that do not use designs and polynomial-based error-correcting codes and that are very simple and efficient. For example, one extractor produces each output bit separately in O(log‚Ā°2n)O(\log^2 n) time. These extractors work for weak sources with min entropy őĽn\lambda n, for arbitrary constant őĽ>0\lambda > 0, have seed length O(log‚Ā°2n)O(\log^2 n), and their output length is ‚ČąnőĽ/3\approx n^{\lambda/3}.Comment: 21 pages, an extended abstract will appear in Proc. ICALP 2005; small corrections, some comments and references adde

    Dynamical analysis of the cluster pair: A3407 + A3408

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    We carried out a dynamical study of the galaxy cluster pair A3407 \& A3408 based on a spectroscopic survey obtained with the 4 meter Blanco telescope at the CTIO, plus 6dF data, and ROSAT All-Sky-Survey. The sample consists of 122 member galaxies brighter than mR=20m_R=20. Our main goal is to probe the galaxy dynamics in this field and verify if the sample constitutes a single galaxy system or corresponds to an ongoing merging process. Statistical tests were applied to clusters members showing that both the composite system A3407 + A3408 as well as each individual cluster have Gaussian velocity distribution. A velocity gradient of ‚ąľ847¬Ī114\sim 847\pm 114 km‚ÄÖ‚Ääs‚ąí1\rm km\;s^{-1} was identified around the principal axis of the projected distribution of galaxies, indicating that the global field may be rotating. Applying the KMM algorithm to the distribution of galaxies we found that the solution with two clusters is better than the single unit solution at the 99\% c.l. This is consistent with the X-ray distribution around this field, which shows no common X-ray halo involving A3407 and A3408. We also estimated virial masses and applied a two-body model to probe the dynamics of the pair. The more likely scenario is that in which the pair is gravitationally bound and probably experiences a collapse phase, with the cluster cores crossing in less than ‚ąľ\sim1 h‚ąí1h^{-1} Gyr, a pre-merger scenario. The complex X-ray morphology, the gas temperature, and some signs of galaxy evolution in A3408 suggests a post-merger scenario, with cores having crossed each other ‚ąľ1.65h‚ąí1\sim 1.65 h^{-1}Gyr ago, as an alternative solution.Comment: 17 pages, 12 figures, submitted to MNRAS, accepted 2016 May 9. Received 2016 May 9; in original form 2016 April 1

    Prognose do crescimento volumétrico de espécies de Eucalyptus para a Região Centro-Sul do Brasil.

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    Relatam-se os resultados obtidos para a prognose do crescimento volumetrico de Eucalyptus deanei, E. viminalis, E. globulus var. bicostata, E. cambiju e E. nitens, implantados na regiao de Ponta Grossa, PR. Dois modelos, comprovadamente adequados, foram testados com esse objetivo: i. metodo das regressoes sucessivas, desenvolvido por Ferreira (1978) e melhorado por Guimaraes e Castro (1982) e Ferreira (1989); ii. funcao sigmoidal de Chapman-Richards, de dois parametros,com diversas aplicacoes no campo florestal. Apos as analises efetuadas, apresentam-se as seguintes conclusoes: 1. O modelo das regressoes sucessivas, com a vantagem de nao necessitar da serie historica completa dos volumes para uma mesma parcela para ser ajustada, tem precisao satisfatoria para os objetivos deste trabalho. 2. Os melhores resultados, expressos pelos menores erros da projecao volumetrica, foram obtidos com o modelo de Chapman - Richards. 3. A idade base para tres anos de idade. 4. Nao foi possivel ajustar-se os modelos testados aos dados de E. nitens devido a inadaptacao da especie a regiao de Ponta Grossa. Nessa especie observou-se uma acentuada queda de sobrevivencia ao longo da rotacao. 5. Para o modelo de regressoes sucessivas, a mesma equacao pode ser utilizada para todas as especies, com base no volume obtido ao quarto ano

    Estimating the Volume of Unknown Inclusions in an Electrically Conducting Body with Voltage Measurements

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    We propose a novel technique to estimate the total volume of unknown insulating inclusions in an electrically conducting body from voltage measurements. Unlike conventional Electrical Impedance Tomography (EIT) systems that usually exhibit low spatial resolution and accuracy, the proposed device is composed of a pair of driving electrodes which, supplied with a known sinusoidal voltage, create a current density field inside a region of interest. The electrodes are designed to generate a current density field in the region of interest that is uniform, to a good approximation, when the inclusions are not present. A set of electrodes with a polygonal geometry is used for four-wires resistance measurements. The proposed technique has been tested designing a low cost prototype, where all electrodes are on the bottom of the conducting body, showing good performances. Such a device may be used to monitor the volume of biological cells inside cell culture dishes or the volume of blood clots in micro-channels in lab-on-a-chip biosensor

    On the Creation of the Universe out of Nothing

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    We explain how the Universe was created with no expenditure of energy or initial mass.Comment: To be presented at IWARA 2009 (4th International Workshop on Astronomy and Relativistic Astrophysics), to be held in Brazil, October 200
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