12,268 research outputs found

    Probability distributions and Gleason's Theorem

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    We discuss concrete examples for frame functions and their associated density operators, as well as for non-Gleason type probability measures.Comment: Presented at the 4th conference on Foundations of Probability and Physics, held in Vaexjoe, June 4-9, 200

    A small-scale dynamo in feedback-dominated galaxies - III. Cosmological simulations

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    Magnetic fields are widely observed in the Universe in virtually all astrophysical objects, from individual stars to entire galaxies, even in the intergalactic medium, but their specific generation has long been debated. Due to the development of more realistic models of galaxy formation, viable scenarios are emerging to explain cosmic magnetism, thanks to both deeper observations and more efficient and accurate computer simulations. We present here a new cosmological high-resolution zoom-in magnetohydrodynamic (MHD) simulation, using the adaptive mesh refinement (AMR) technique, of a dwarf galaxy with an initially weak and uniform magnetic seed field that is amplified by a small-scale dynamo driven by supernova-induced turbulence. As first structures form from the gravitational collapse of small density fluctuations, the frozen-in magnetic field separates from the cosmic expansion and grows through compression. In a second step, star formation sets in and establishes a strong galactic fountain, self-regulated by supernova explosions. Inside the galaxy, the interstellar medium becomes highly turbulent, dominated by strong supersonic shocks, as demonstrated by the spectral analysis of the gas kinetic energy. In this turbulent environment, the magnetic field is quickly amplified via a small-scale dynamo process and is finally carried out into the circumgalactic medium by a galactic wind. This realistic cosmological simulation explains how initially weak magnetic seed fields can be amplified quickly in early, feedback-dominated galaxies, and predicts, as a consequence of the small scale dynamo process, that high-redshift magnetic fields are likely to be dominated by their small scale components.Comment: 6 pages, 6 figures, submitted to MNRA

    Average optimality for continuous-time Markov decision processes in polish spaces

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    This paper is devoted to studying the average optimality in continuous-time Markov decision processes with fairly general state and action spaces. The criterion to be maximized is expected average rewards. The transition rates of underlying continuous-time jump Markov processes are allowed to be unbounded, and the reward rates may have neither upper nor lower bounds. We first provide two optimality inequalities with opposed directions, and also give suitable conditions under which the existence of solutions to the two optimality inequalities is ensured. Then, from the two optimality inequalities we prove the existence of optimal (deterministic) stationary policies by using the Dynkin formula. Moreover, we present a ``semimartingale characterization'' of an optimal stationary policy. Finally, we use a generalized Potlach process with control to illustrate the difference between our conditions and those in the previous literature, and then further apply our results to average optimal control problems of generalized birth--death systems, upwardly skip-free processes and two queueing systems. The approach developed in this paper is slightly different from the ``optimality inequality approach'' widely used in the previous literature.Comment: Published at http://dx.doi.org/10.1214/105051606000000105 in the Annals of Applied Probability (http://www.imstat.org/aap/) by the Institute of Mathematical Statistics (http://www.imstat.org

    Density of states at disorder-induced phase transitions in a multichannel Majorana wire

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    An NN-channel spinless p-wave superconducting wire is known to go through a series of NN topological phase transitions upon increasing the disorder strength. Here, we show that at each of those transitions the density of states shows a Dyson singularity ν(ε)ε1lnε3\nu(\varepsilon) \propto \varepsilon^{-1}|\ln\varepsilon|^{-3} , whereas ν(ε)εα1\nu(\varepsilon) \propto \varepsilon^{|\alpha|-1} has a power-law singularity for small energies ε\varepsilon away from the critical points. Using the concept of "superuniversality" [Gruzberg, Read, and Vishveshwara, Phys. Rev. B 71, 245124 (2005)], we are able to relate the exponent α\alpha to the wire's transport properties at zero energy and, hence, to the mean free path ll and the superconducting coherence length ξ\xi.Comment: 4+1 pages, 3 figure

    What drives Venture Capital Syndication

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    This paper analyses the syndication behavior of VC organisations and the factors influencing their overall propensity to co-invest. We develop hypothesis concerning the investment behavior of Venture Capitalists in the German market and compare these hypothesis to the actual empirical evidence from a data set including 2,500 VC investments. We find that the underlying theories of financial and resource driven motives can indeed be used to explain the observed behavior for syndicated venture capital investments. We show that mainly Resource driven motives foster the propensity to syndicate an investment. Additionally, we find that Venture Capital Firms tend to diversify their portfolio, such that both motives of venture capital syndication (Finance and Resource driven) seem to be present at the same time and play a significant role simultaneously for the decision to jointly co-invest. We find evidence that a lower level of experience and expertise fosters the need to syndicate an investment.Venture Capital, Syndication