5,940 research outputs found

    Dynamics of abelian subgroups of GL(n, C): a structure's Theorem

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    In this paper, we characterize the dynamic of every abelian subgroups G\mathcal{G} of GL(nn, K\mathbb{K}), K=R\mathbb{K} = \mathbb{R} or C\mathbb{C}. We show that there exists a G\mathcal{G}-invariant, dense open set UU in Kn\mathbb{K}^{n} saturated by minimal orbits with KnU\mathbb{K}^{n}- U a union of at most nn G\mathcal{G}-invariant vectorial subspaces of Kn\mathbb{K}^{n} of dimension n1n-1 or n2n-2 on K\mathbb{K}. As a consequence, G\mathcal{G} has height at most nn and in particular it admits a minimal set in Kn{0}\mathbb{K}^{n}-\{0\}.Comment: 16 page

    LINEAR FEATURES IN PHOTOGRAMMETRY

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    Traditional photogrammetric activities such as orientation, triangulation, and object space reconstruction have been relying on distinct points in their underlying operations. With the evolution of digital photogrammetry, there has been a tremendous interest in utilizing linear features in various photogrammetric activities. This interest has been motivated by the fact that the extraction of linear features from the image space is easier to automate than distinct points. On the other hand, object space linear features can be directly derived form terrestrial Mobile Mapping Systems (MMS), GIS databases, and/or existing maps. Moreover, automatic matching of linear features, either within overlapping images or between image and object space, is easier than that of distinct points. Finally, linear features possess more semantic information than distinct points since they most probably correspond to object boundaries. Such semantics can be automatically identified in imagery to facilitate higher-level tasks (e.g., surface reconstruction and object recognition). This paper summarizes the use of linear features, which might be represented by analytical functions (e.g., straight-line segments) or irregular (freeform) shapes, in photogrammetric activities such as automatic space resection, photogrammetric triangulation, camera calibration, image matching, surface reconstruction, image-to-image registration, and absolute orientation. Current progress, future expectations, and possible research directions are discussed as well

    Autologous Bone Marrow Stem Cells in the Treatment of Chronic Liver Disease

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    Chronic liver disease (CLD) is increasing worldwide yet there has been no major advance in effective therapies for almost five decades. There is mounting evidence that adult haematopoietic stem cells (HSC) are capable of differentiating into many types of tissue, including skeletal and cardiac muscle, neuronal cells, pneumocytes and hepatocytes. These recent advances in regenerative medicine have brought hope for patients with liver cirrhosis awaiting transplantation. New findings in adult stem cell biology are transforming our understanding of tissue repair raising hopes of successful regenerative hepatology. Although all clinical trials to date have shown some improvement in liver function and CD34(+) cells have been used safely for BM transplantation for over 20 years, only randomised controlled clinical trials will be able to fully assess the potential clinical benefit of adult stem cell therapy for patients with CLD. This article focuses on the potential of bone marrow stem cells (BMSCs) in the management of CLD and the unresolved issues regarding their role. We also outline the different mechanisms by which stem cells may impact on liver disease

    Chaos in Time Dependent Variational Approximations to Quantum Dynamics

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    Dynamical chaos has recently been shown to exist in the Gaussian approximation in quantum mechanics and in the self-consistent mean field approach to studying the dynamics of quantum fields. In this study, we first show that any variational approximation to the dynamics of a quantum system based on the Dirac action principle leads to a classical Hamiltonian dynamics for the variational parameters. Since this Hamiltonian is generically nonlinear and nonintegrable, the dynamics thus generated can be chaotic, in distinction to the exact quantum evolution. We then restrict attention to a system of two biquadratically coupled quantum oscillators and study two variational schemes, the leading order large N (four canonical variables) and Hartree (six canonical variables) approximations. The chaos seen in the approximate dynamics is an artifact of the approximations: this is demonstrated by the fact that its onset occurs on the same characteristic time scale as the breakdown of the approximations when compared to numerical solutions of the time-dependent Schrodinger equation.Comment: 10 pages (12 figures), RevTeX (plus macro), uses epsf, minor typos correcte

    Stochastic Inflation:The Quantum Phase Space Approach

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    In this paper a quantum mechanical phase space picture is constructed for coarse-grained free quantum fields in an inflationary Universe. The appropriate stochastic quantum Liouville equation is derived. Explicit solutions for the phase space quantum distribution function are found for the cases of power law and exponential expansions. The expectation values of dynamical variables with respect to these solutions are compared to the corresponding cutoff regularized field theoretic results (we do not restrict ourselves only to \VEV{\F^2}). Fair agreement is found provided the coarse-graining scale is kept within certain limits. By focusing on the full phase space distribution function rather than a reduced distribution it is shown that the thermodynamic interpretation of the stochastic formalism faces several difficulties (e.g., there is no fluctuation-dissipation theorem). The coarse-graining does not guarantee an automatic classical limit as quantum correlations turn out to be crucial in order to get results consistent with standard quantum field theory. Therefore, the method does {\em not} by itself constitute an explanation of the quantum to classical transition in the early Universe. In particular, we argue that the stochastic equations do not lead to decoherence.Comment: 43 page

    Time evolution of correlation functions and thermalization

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    We investigate the time evolution of a classical ensemble of isolated periodic chains of O(N)-symmetric anharmonic oscillators. Our method is based on an exact evolution equation for the time dependence of correlation functions. We discuss its solutions in an approximation which retains all contributions in next-to-leading order in a 1/N expansion and preserves time reflection symmetry. We observe effective irreversibility and approximate thermalization. At large time the system approaches stationary solutions in the vicinity of, but not identical to, thermal equilibrium. The ensemble therefore retains some memory of the initial condition beyond the conserved total energy. Such a behavior with incomplete thermalization is referred to as "mesoscopic dynamics". It is expected for systems in a small volume. Surprisingly, we find that the nonthermal asymptotic stationary solutions do not change for large volume. This raises questions on Boltzmann's conjecture that macroscopic isolated systems thermalize.Comment: 40 pages, 9 figure

    Scalable Production of Ambient Stable Hybrid Bismuth-based Materials: AACVD of Phenethylammonium Bismuth Iodide Films

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    Large homogeneous and adherent coatings of phenethylammonium bismuth iodide were produced using the cost-effective and scalable aerosol-assisted chemical vapour deposition (AACVD) methodology. The film morphology was found to depend on the deposition conditions and substrates, resulting in different optical properties to those reported from their spin-coated counterparts. Optoelectronic characterization revealed band bending effects occurring between the hybrid material and semiconducting substrates (TiO2 and FTO) due to heterojunction formation, and the optical bandgap of the hybrid material was calculated from UV-visible and PL spectrometry to be 2.05 eV. Maximum values for hydrophobicity and crystallographic preferential orientation were observed for films deposited on FTO/glass substrates, closely followed by values from films deposited on TiO2/glass substrates

    NLC-2 graph recognition and isomorphism

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    NLC-width is a variant of clique-width with many application in graph algorithmic. This paper is devoted to graphs of NLC-width two. After giving new structural properties of the class, we propose a O(n2m)O(n^2 m)-time algorithm, improving Johansson's algorithm \cite{Johansson00}. Moreover, our alogrithm is simple to understand. The above properties and algorithm allow us to propose a robust O(n2m)O(n^2 m)-time isomorphism algorithm for NLC-2 graphs. As far as we know, it is the first polynomial-time algorithm.Comment: soumis \`{a} WG 2007; 12

    Exact Thermodynamics of the Double sinh-Gordon Theory in 1+1-Dimensions

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    We study the classical thermodynamics of a 1+1-dimensional double-well sinh-Gordon theory. Remarkably, the Schrodinger-like equation resulting from the transfer integral method is quasi-exactly solvable at several temperatures. This allows exact calculation of the partition function and some correlation functions above and below the short-range order (``kink'') transition, in striking agreement with high resolution Langevin simulations. Interesting connections with the Landau-Ginzburg and double sine-Gordon models are also established.Comment: 4 pages, 3 figures (embedded using epsf), uses RevTeX plus macro (included). Minor revision to match journal version, Phys. Rev. Lett. (in press
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