1,091 research outputs found

    A comparison of incompressible limits for resistive plasmas

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    The constraint of incompressibility is often used to simplify the magnetohydrodynamic (MHD) description of linearized plasma dynamics because it does not affect the ideal MHD marginal stability point. In this paper two methods for introducing incompressibility are compared in a cylindrical plasma model: In the first method, the limit γ\gamma \to \infty is taken, where γ\gamma is the ratio of specific heats; in the second, an anisotropic mass tensor ρ\mathbf{\rho} is used, with the component parallel to the magnetic field taken to vanish, ρ0\rho_{\parallel} \to 0. Use of resistive MHD reveals the nature of these two limits because the Alfv\'en and slow magnetosonic continua of ideal MHD are converted to point spectra and moved into the complex plane. Both limits profoundly change the slow-magnetosonic spectrum, but only the second limit faithfully reproduces the resistive Alfv\'en spectrum and its wavemodes. In ideal MHD, the slow magnetosonic continuum degenerates to the Alfv\'en continuum in the first method, while it is moved to infinity by the second. The degeneracy in the first is broken by finite resistivity. For numerical and semi-analytical study of these models, we choose plasma equilibria which cast light on puzzling aspects of results found in earlier literature.Comment: 14 pages, 10 figure

    Searching of gapped repeats and subrepetitions in a word

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    A gapped repeat is a factor of the form uvuuvu where uu and vv are nonempty words. The period of the gapped repeat is defined as u+v|u|+|v|. The gapped repeat is maximal if it cannot be extended to the left or to the right by at least one letter with preserving its period. The gapped repeat is called α\alpha-gapped if its period is not greater than αv\alpha |v|. A δ\delta-subrepetition is a factor which exponent is less than 2 but is not less than 1+δ1+\delta (the exponent of the factor is the quotient of the length and the minimal period of the factor). The δ\delta-subrepetition is maximal if it cannot be extended to the left or to the right by at least one letter with preserving its minimal period. We reveal a close relation between maximal gapped repeats and maximal subrepetitions. Moreover, we show that in a word of length nn the number of maximal α\alpha-gapped repeats is bounded by O(α2n)O(\alpha^2n) and the number of maximal δ\delta-subrepetitions is bounded by O(n/δ2)O(n/\delta^2). Using the obtained upper bounds, we propose algorithms for finding all maximal α\alpha-gapped repeats and all maximal δ\delta-subrepetitions in a word of length nn. The algorithm for finding all maximal α\alpha-gapped repeats has O(α2n)O(\alpha^2n) time complexity for the case of constant alphabet size and O(nlogn+α2n)O(n\log n + \alpha^2n) time complexity for the general case. For finding all maximal δ\delta-subrepetitions we propose two algorithms. The first algorithm has O(nloglognδ2)O(\frac{n\log\log n}{\delta^2}) time complexity for the case of constant alphabet size and O(nlogn+nloglognδ2)O(n\log n +\frac{n\log\log n}{\delta^2}) time complexity for the general case. The second algorithm has O(nlogn+nδ2log1δ)O(n\log n+\frac{n}{\delta^2}\log \frac{1}{\delta}) expected time complexity

    Continuity and change - The planning and management of long distance walking routes in Scotland

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    In recent years a number of changes have taken place in Scotland in respect of issues of land management, access and the natural environment. These include the creation of Scotland’s first National Parks in 2002 and the introduction of the Land Reform (Scotland) Act 2003, which has enshrined in legislation the principle of responsible access in the countryside. The aim of this study was to consider the implications of these changes for a specific type of recreational land use in Scotland, Long Distance (Walking) Routes (LDRs). Using semi-structured interviews with representatives of a number of agencies and with other individuals closely involved with LDRs, the research considered the extent to which these changes have or may alter the rationale for the provision of LDRs, their funding and their management. The research indicates a need and a willingness to build on existing stakeholder approaches to management with a view to engaging a broader range of communities of interest. The main challenge for those involved with LDRs is how to fund future development of these routes. One aim of a more participatory stakeholder management approach is to help route managers to use public funds to lever funds from other source

    Responsibility modelling for civil emergency planning

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    This paper presents a new approach to analysing and understanding civil emergency planning based on the notion of responsibility modelling combined with HAZOPS-style analysis of information requirements. Our goal is to represent complex contingency plans so that they can be more readily understood, so that inconsistencies can be highlighted and vulnerabilities discovered. In this paper, we outline the framework for contingency planning in the United Kingdom and introduce the notion of responsibility models as a means of representing the key features of contingency plans. Using a case study of a flooding emergency, we illustrate our approach to responsibility modelling and suggest how it adds value to current textual contingency plans

    Path Integral Monte Carlo Simulation of the Low-Density Hydrogen Plasma

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    Restricted path integral Monte Carlo simulations are used to calculate the equilibrium properties of hydrogen in the density and temperature range of 9.83×104ρ0.153gcm39.83 \times 10^{-4}\rm \leq \rho \leq 0.153 \rm gcm^{-3} and 5000T250000K5000 \leq T \leq 250 000 \rm K. We test the accuracy of the pair density matrix and analyze the dependence on the system size, on the time step of the path integral and on the type of nodal surface. We calculate the equation of state and compare with other models for hydrogen valid in this regime. Further, we characterize the state of hydrogen and describe the changes from a plasma to an atomic and molecular liquid by analyzing the pair correlation functions and estimating the number of atoms and molecules present.Comment: 12 pages, 21 figures, submitted for Phys. Rev.

    3-Acetyl-6-chloro-1-ethyl-4-phenyl­quinolin-2(1H)-one

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    In the title compound, C19H16ClNO2, the dihedral angle between the plane of the phenyl substituent and 3-acetyl­quinoline unit is 75.44 (5)°. The crystal structure is stabilized by inter­molecular C—H⋯O hydrogen bond

    Cisternal Organization of the Endoplasmic Reticulum during Mitosis

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    The endoplasmic reticulum (ER) of animal cells is a single, dynamic, and continuous membrane network of interconnected cisternae and tubules spread out throughout the cytosol in direct contact with the nuclear envelope. During mitosis, the nuclear envelope undergoes a major rearrangement, as it rapidly partitions its membrane-bound contents into the ER. It is therefore of great interest to determine whether any major transformation in the architecture of the ER also occurs during cell division. We present structural evidence, from rapid, live-cell, three-dimensional imaging with confirmation from high-resolution electron microscopy tomography of samples preserved by high-pressure freezing and freeze substitution, unambiguously showing that from prometaphase to telophase of mammalian cells, most of the ER is organized as extended cisternae, with a very small fraction remaining organized as tubules. In contrast, during interphase, the ER displays the familiar reticular network of convolved cisternae linked to tubules

    Towards a better solution to the shortest common supersequence problem: the deposition and reduction algorithm

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    BACKGROUND: The problem of finding a Shortest Common Supersequence (SCS) of a set of sequences is an important problem with applications in many areas. It is a key problem in biological sequences analysis. The SCS problem is well-known to be NP-complete. Many heuristic algorithms have been proposed. Some heuristics work well on a few long sequences (as in sequence comparison applications); others work well on many short sequences (as in oligo-array synthesis). Unfortunately, most do not work well on large SCS instances where there are many, long sequences. RESULTS: In this paper, we present a Deposition and Reduction (DR) algorithm for solving large SCS instances of biological sequences. There are two processes in our DR algorithm: deposition process, and reduction process. The deposition process is responsible for generating a small set of common supersequences; and the reduction process shortens these common supersequences by removing some characters while preserving the common supersequence property. Our evaluation on simulated data and real DNA and protein sequences show that our algorithm consistently produces the best results compared to many well-known heuristic algorithms, and especially on large instances. CONCLUSION: Our DR algorithm provides a partial answer to the open problem of designing efficient heuristic algorithm for SCS problem on many long sequences. Our algorithm has a bounded approximation ratio. The algorithm is efficient, both in running time and space complexity and our evaluation shows that it is practical even for SCS problems on many long sequences
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