676 research outputs found

    Imprints of magnetic power and helicity spectra on radio polarimetry statistics

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    Statistical properties of turbulent magnetic fields in radio-synchrotron sources should imprint on the statistics of polarimetric observables. In search of these imprints, we calculate correlation and cross-correlation functions from a set of observables containing the total intensity I, the polarized intensity P and the Faraday depth phi. The correlation functions are evaluated for all combinations of observables up to fourth order in the magnetic field B. We derive these as far as possible analytically and from first principles only using some basic assumptions such as Gaussian statistics of the underlying magnetic field in the observed region and statistical homogeneity. We further assume some simplifications to reduce the complexity of the calculations, as for a start we were interested in a proof of concept. Using this statistical approach, we show that it is in principle possible to gain information about the helical part of the magnetic power spectrum, namely via the correlation functions and . Using this insight, we construct an easy-to-use test for helicity, called LITMUS (Local Inference Test for Magnetic fields which Uncovers heliceS). For now, all calculations are given in a Faraday-free case, but set up in a way so that Faraday rotational effects could be included later on.Comment: 24 pages, 4 figures; typos corrected; additional explanations in section 1 and 2; revised and extended derivation in section 5, results unchange

    Advances in Proof-Theoretic Semantics

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    Logic; Mathematical Logic and Foundations; Mathematical Logic and Formal Language

    Proof Transformation with Built-in Equality Predicate

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    One of the main reasons why computer generated proofs are not widely accepted is often their complexity and incomprehensibility. Especially proofs of mathematical theorems with equations are normally presented in an inadequate and not intuitive way. This is even more of a problem for the presentation of inferences drawn by automated reasoning components in other AI systems. For first order logic, proof transformation procedures have been designed in order to structure proofs and state them in a formalism that is more familiar to human mathematicians. In this report we generalize these approaches, so that proofs involving equational reasoning can also be handled. To this end extended refutation graphs are introduced to represent combined resolution and paramodulation proofs. In the process of transforming these proofs into natural deduction proofs with equality, the inherent structure can also be extracted by exploiting topological properties of refutation graphs

    How tax incentives affect decisions to invest in developing countries

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    The authors contend that in evaluating and designing investment incentives in developing economies, analysts should consider their effect on: the marginal effective tax rate (METR). Even simple tax incentives can perversely affect the METR. Many schemes have relatively generous write-offs to begin with, so generous that a negative marginal effective tax rate is not uncommon. In these circumstances, tax rate reductions (including tax holidays) can discourage investment. Investment tax credits are more likely to be effective. Loss firms. Incentives that do not have generous loss-offsetting or refundability provisions willbe of limited use to firms likely to suffer losses (including small growing firms and firms in risky environments). Cash flows. Incentives that improve firms'cash flows may be more effective than those that do not. Refundability may be important here. Simply adopting cash-flow costing principles with refundability may be more effective than reducing tax rates. Foreign-owned firms. If the value of a tax incentive is fully offset by reduced credits for foreign taxes, the incentive effect will probably be minimal. Capital allocation among assets. Some measures favor short- over long-lived capital, machinery over inventory, some industries over others. Incentives that encourage investment selectively may cause distortions in the way capital is allocated. Other factors to be considered in designing tax incentives: inflation, which is typically high in developing economies. Incentives should offset the effects of inflation; tax evasion, a common problem in developing countries; technology transfer; the fulfillment of social, environmental, and regional non-economic objectives; the effects on firms'organization (do the incentives encourage mergers, takeovers, or bankruptcy?)Economic Theory&Research,Environmental Economics&Policies,International Terrorism&Counterterrorism,Public Sector Economics&Finance,Banks&Banking Reform

    Denotational proof languages

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    Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2000.Includes bibliographical references (p. [417]-421).by Konstantinos Arkoudas.Ph.D
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