20,142 research outputs found

    The role of sign in students' modeling of scalar equations

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    We describe students revising the mathematical form of physics equations to match the physical situation they are describing, even though their revision violates physical laws. In an unfamiliar air resistance problem, a majority of students in a sophomore level mechanics class at some point wrote Newton's Second Law as F = -ma; they were using this form to ensure that the sign of the force pointed in a direction consistent with the chosen coordinate system while assuming that some variables have only positive value. We use one student's detailed explanation to suggest that students' issues with variables are context-dependent, and that much of their reasoning is useful for productive instruction.Comment: 5 pages, 1 figure, to be published in The Physics Teache

    Exploring harm in psychotherapy: Perspectives of clinicians working with children and young people

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    Aims: The potential for harm to occur from talking therapies has been acknowledged in academic literature. However, there is a paucity of research when it comes to exploring this phenomenon when working with young patients. This study explores clinicians’ perspectives on harm from talking therapies when working with children and young people. / Method: Eleven clinicians were interviewed on the types of harm that could occur from talking therapies, as well as the potential mechanisms. Data were analysed inductively using thematic analysis. / Results: Two themes were identified around types of harm: ‘clinical deterioration’ and ‘retraumatisation’. Additionally, four groups of mechanisms were identified: ‘Administrative factors’, ‘Relationship factors’, ‘Therapist factors’ and ‘Contextual factors’. / Discussion: Clinicians are able to identify some specific types of harm when working with children and young people and understand how these could occur. The clinical implications of these findings are explored, along with limitations and directions for future research

    Travelling Salesman Problem with a Center

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    We study a travelling salesman problem where the path is optimized with a cost function that includes its length LL as well as a certain measure CC of its distance from the geometrical center of the graph. Using simulated annealing (SA) we show that such a problem has a transition point that separates two phases differing in the scaling behaviour of LL and CC, in efficiency of SA, and in the shape of minimal paths.Comment: 4 pages, minor changes, accepted in Phys.Rev.

    Fully three dimensional breather solitons can be created using Feshbach resonance

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    We investigate the stability properties of breather solitons in a three-dimensional Bose-Einstein Condensate with Feshbach Resonance Management of the scattering length and con ned only by a one dimensional optical lattice. We compare regions of stability in parameter space obtained from a fully 3D analysis with those from a quasi two-dimensional treatment. For moderate con nement we discover a new island of stability in the 3D case, not present in the quasi 2D treatment. Stable solutions from this region have nontrivial dynamics in the lattice direction, hence they describe fully 3D breather solitons. We demonstrate these solutions in direct numerical simulations and outline a possible way of creating robust 3D solitons in experiments in a Bose Einstein Condensate in a one-dimensional lattice. We point other possible applications.Comment: 4 pages, 4 figures; accepted to Physical Review Letter

    Relativistic quantum plasma dispersion functions

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    Relativistic quantum plasma dispersion functions are defined and the longitudinal and transverse response functions for an electron (plus positron) gas are written in terms of them. The dispersion is separated into Landau-damping, pair-creation and dissipationless regimes. Explicit forms are given for the RQPDFs in the cases of a completely degenerate distribution and a nondegenerate thermal (J\"uttner) distribution. Particular emphasis is placed on the relation between dissipation and dispersion, with the dissipation treated in terms of the imaginary parts of RQPDFs. Comparing the dissipation calculated in this way with the existing treatments leads to the identification of errors in the literature, which we correct. We also comment on a controversy as to whether the dispersion curves in a superdense plasma pass through the region where pair creation is allowed.Comment: 16 pages, 1 figur

    Identifying influential spreaders and efficiently estimating infection numbers in epidemic models: a walk counting approach

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    We introduce a new method to efficiently approximate the number of infections resulting from a given initially-infected node in a network of susceptible individuals. Our approach is based on counting the number of possible infection walks of various lengths to each other node in the network. We analytically study the properties of our method, in particular demonstrating different forms for SIS and SIR disease spreading (e.g. under the SIR model our method counts self-avoiding walks). In comparison to existing methods to infer the spreading efficiency of different nodes in the network (based on degree, k-shell decomposition analysis and different centrality measures), our method directly considers the spreading process and, as such, is unique in providing estimation of actual numbers of infections. Crucially, in simulating infections on various real-world networks with the SIR model, we show that our walks-based method improves the inference of effectiveness of nodes over a wide range of infection rates compared to existing methods. We also analyse the trade-off between estimate accuracy and computational cost, showing that the better accuracy here can still be obtained at a comparable computational cost to other methods.Comment: 6 page
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