11,644 research outputs found
Electrostriction and guidance of sound by light in optical fibers
We investigate the generation of phonon wavepackets in optical fibers via
electrostriction from coherent optical waves. Solving the elastodynamic
equation subject to the electrostrictive force, we are able to reproduce
experimental spectra found in standard and photonic crystal fibers. We discuss
the two important practical cases of forward interaction, dominated by elastic
resonances of the fiber, and backward interaction, for which an efficient
mechanism of phonon guidance by light is found. The latter result describes the
formation of the coherent phonon wavepacket involved in stimulated Brillouin
scattering
A statistical analysis of particle trajectories in living cells
Recent advances in molecular biology and fluorescence microscopy imaging have
made possible the inference of the dynamics of single molecules in living
cells. Such inference allows to determine the organization and function of the
cell. The trajectories of particles in the cells, computed with tracking
algorithms, can be modelled with diffusion processes. Three types of diffusion
are considered : (i) free diffusion; (ii) subdiffusion or (iii) superdiffusion.
The Mean Square Displacement (MSD) is generally used to determine the different
types of dynamics of the particles in living cells (Qian, Sheetz and Elson
1991). We propose here a non-parametric three-decision test as an alternative
to the MSD method. The rejection of the null hypothesis -- free diffusion -- is
accompanied by claims of the direction of the alternative (subdiffusion or a
superdiffusion). We study the asymptotic behaviour of the test statistic under
the null hypothesis, and under parametric alternatives which are currently
considered in the biophysics literature, (Monnier et al,2012) for example. In
addition, we adapt the procedure of Benjamini and Hochberg (2000) to fit with
the three-decision test setting, in order to apply the test procedure to a
collection of independent trajectories. The performance of our procedure is
much better than the MSD method as confirmed by Monte Carlo experiments. The
method is demonstrated on real data sets corresponding to protein dynamics
observed in fluorescence microscopy.Comment: Revised introduction. A clearer and shorter description of the model
(section 2
Global Contests in the Production of Business Knowledge :
Drawing on institutional theory, the global production of business research is analysed by examining the system of written outputs using one of the largest databases of journal papers ever assembled, covering over 65,000 articles produced by more than 54,000 authors from over 8,000 different institutions across the period 1992-2005. We begin by pointing out how the US business schools pioneered the modern institutional system of undertaking and disseminating research that involves the intertwining of and university business schools and journals. While Wharton and Harvard are still the leading universities globally, their crowns are slipping, together with the position of the US generally. We observe the greatest challenges to the existing order as coming from European and Asian institutions that have either copied, or been inspired to innovate by adapting, the US system. London Business School, Erasmus, INSEAD and Tilburg are threatening to topple leading US universities in the undertaking of research, and other European and Asian institutions are close behind.
Decidability and Universality in Symbolic Dynamical Systems
Many different definitions of computational universality for various types of
dynamical systems have flourished since Turing's work. We propose a general
definition of universality that applies to arbitrary discrete time symbolic
dynamical systems. Universality of a system is defined as undecidability of a
model-checking problem. For Turing machines, counter machines and tag systems,
our definition coincides with the classical one. It yields, however, a new
definition for cellular automata and subshifts. Our definition is robust with
respect to initial condition, which is a desirable feature for physical
realizability.
We derive necessary conditions for undecidability and universality. For
instance, a universal system must have a sensitive point and a proper
subsystem. We conjecture that universal systems have infinite number of
subsystems. We also discuss the thesis according to which computation should
occur at the `edge of chaos' and we exhibit a universal chaotic system.Comment: 23 pages; a shorter version is submitted to conference MCU 2004 v2:
minor orthographic changes v3: section 5.2 (collatz functions) mathematically
improved v4: orthographic corrections, one reference added v5:27 pages.
Important modifications. The formalism is strengthened: temporal logic
replaced by finite automata. New results. Submitte
Learning Local Receptive Fields and their Weight Sharing Scheme on Graphs
We propose a simple and generic layer formulation that extends the properties
of convolutional layers to any domain that can be described by a graph. Namely,
we use the support of its adjacency matrix to design learnable weight sharing
filters able to exploit the underlying structure of signals in the same fashion
as for images. The proposed formulation makes it possible to learn the weights
of the filter as well as a scheme that controls how they are shared across the
graph. We perform validation experiments with image datasets and show that
these filters offer performances comparable with convolutional ones.Comment: To appear in 2017, 5th IEEE Global Conference on Signal and
Information Processing, 5 pages, 3 figures, 3 table
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