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21.
Roberto Bassani 《Meccanica》2006,41(4):375-389
S. Earnshaw, a mathematical physicist born in England two centuries ago, was the author of studies on the transmission of
sound and light. His theory on the transmission of light by “detached particles” has also influenced some studies on the stability
of bodies in potential fields, particularly studies on the levitation of permanent magnets, i.e. on passive magnetic levitation.
This theorem was so well known that even Maxwell mentioned it, and is today cited almost as an axiom in the ambit of passive
magnetization. However, what is less well known is that the theory actually only relates to magnetostatics. In this paper,
the mathematic model of the theory that refers to stability is outlined, along with some works that refer to it, in particular
those on passive magnetic levitation. It is then pointed out that spaces of low instability exist in magnetostatics, and that
stability may exist in the magneto-stationary and in magneto-dynamic ambit. 相似文献
22.
Roberto Ballini Dennis Fiorini Maria Victoria Gil Alessandro Palmieri 《Tetrahedron》2004,60(12):2799-2804
Nitroalkanes have been found to give good yields in Michael and nitroaldol (Henry) reactions by the use of a catalytic amount (10 mol%) of CTAOH, at room temperature and under solvent free conditions and in very short reaction times. The methods do not need a large excess of the nitroalkanes and show good chemoselectivity toward further functionalities. 相似文献
23.
Given a smooth Lagrangian path, both in the finite and in the infinite dimensional (Fredholm) case, we introduce the notion of partial signatures at each isolated intersection of the path with the Maslov cycle. For real-analytic paths, we give a formula for the computation of the Maslov index using the partial signatures; a similar formula holds for the spectral flow of real-analytic paths of Fredholm self-adjoint operators on real separable Hilbert spaces. As applications of the theory, we obtain a semi-Riemannian version of the Morse index theorem for geodesics with possibly conjugate endpoints, and we prove a bifurcation result at conjugate points along semi-Riemannian geodesics. To cite this article: R. Giambò et al., C. R. Acad. Sci. Paris, Ser. I 338 (2004). 相似文献
24.
We review the proof of a conjecture concerning the reality of the spectra of certain PT-symmetric quantum mechanical systems, obtained via a connection between the theories of ordinary differential equations and integrable models. Spectral equivalences inspired by the correspondence are also discussed. 相似文献
25.
Luciano Galeone Roberto Garrappa 《Journal of Computational and Applied Mathematics》1997,80(2):377-195
We study convergence properties of time-point relaxation (TR) Runge-Kutta methods for linear systems of ordinary differential equations. TR methods are implemented by decoupling systems in Gauss-Jacobi, Gauss-Seidel and successive overrelaxation modes (continuous-time iterations) and then solving the resulting subsystems by means of continuous extensions of Runge-Kutta (CRK) methods (discretized iterations). By iterating to convergence, these methods tend to the same limit called diagonally split Runge-Kutta (DSRK) method. We prove that TR methods are equivalent to decouple in the same modes the linear algebraic system obtained by applying DSRK limit method. This issue allows us to study the convergence of TR methods by using standard principles of convergence of iterative methods for linear algebraic systems. For a particular problem regions of convergence are plotted. 相似文献
26.
Roberto Raucci 《Rendiconti del Circolo Matematico di Palermo》1998,47(1):113-128
In this paper a two level optimization problem, in which the problem of inferior level is a Nash-problem with general constraints
(rappresented by multifunctions) and with a unique solution, is considered. On the problem of inferior level approximation
results of the solution are proved, on the problem of superior level existence and approximation results of the solutions
are proved.
相似文献
27.
Jos Vargas Carlos Ramos Roberto D. Zysler Hctor Romero 《Physica B: Condensed Matter》2002,320(1-4):178-180
Chemically prepared (CoxNi1−x)1−yBy (x=0.5, 0.75, 1; y≈0.4) amorphous fine particles were characterized by X-ray diffraction, DTA and TGA, and in situ magnetic measurement as a function of annealing temperature in an inert atmosphere. Magnetic measurement performed in as-prepared and 150°C annealed samples shows an increase of the saturation magnetization and magnetic moment after thermal treatment. Room temperature magnetization increases by factors of 3.5, 1.8, and 1.5, for x=0.5, 0.75, and 1, respectively. These measurements may indicate a local re-ordering of the amorphous phase at temperatures much lower than the full crystallization temperature. 相似文献
28.
Marcos F. de Campos Marília Emura Fernando J.G. Landgraf 《Journal of magnetism and magnetic materials》2006
Electrical steels, when submitted to operation, present continuous decrease of their magnetic properties, depending on the carbon content. This effect is attributed to the increase of the size of carbides, a process also known as coarsening or Ostwald Ripening. Loss separation can offer a better understanding of this phenomenon. Experimental results show that all effect of aging is inside the hysteresis loss component, with the excess losses unaffected. The carbon content in electrical steels should be less than 25 ppm to avoid magnetic aging. 相似文献
29.
30.
We investigate decay properties of correlation functions in a class of chaotic billiards. First we consider the statistics of Poincaré recurrences (induced by a partition of the billiard): the results are in agreement with theoretical bounds by Bunimovich, Sinai, and Bleher, and are consistent with a purely exponential decay of correlations out of marginality. We then turn to the analysis of the velocity-velocity correlation function: except for intermittent situations, the decay is purely exponential, and the decay rates scale in a simple way with the (uniform) curvature of the dispersing arcs. A power-law decay is instead observed when the system is equivalent to an infinite-horizon Lorentz gas. Comments are given on the behaviour of other types of correlation functions, whose decay, during the observed time scale, appears slower than exponential. 相似文献