5,404 research outputs found
Spectrins in Axonal Cytoskeletons: Dynamics Revealed by Extensions and Fluctuations
The macroscopic properties, the properties of individual components and how
those components interact with each other are three important aspects of a
composited structure. An understanding of the interplay between them is
essential in the study of complex systems. Using axonal cytoskeleton as an
example system, here we perform a theoretical study of slender structures that
can be coarse-grained as a simple smooth 3-dimensional curve. We first present
a generic model for such systems based on the fundamental theorem of curves. We
use this generic model to demonstrate the applicability of the well-known
worm-like chain (WLC) model to the network level and investigate the situation
when the system is stretched by strong forces (weakly bending limit). We
specifically studied recent experimental observations that revealed the
hitherto unknown periodic cytoskeleton structure of axons and measured the
longitudinal fluctuations. Instead of focusing on single molecules, we apply
analytical results from the WLC model to both single molecule and network
levels and focus on the relations between extensions and fluctuations. We show
how this approach introduces constraints to possible local dynamics of the
spectrin tetramers in the axonal cytoskeleton and finally suggests simple but
self-consistent dynamics of spectrins in which the spectrins in one spatial
period of axons fluctuate in-sync.Comment: 18 pages, 4 figure
Fermi-Frenet coordinates for space-like curves
We generalize Fermi coordinates, which correspond to an adapted set of
coordinates describing the vicinity of an observer's worldline, to the
worldsheet of an arbitrary spatial curve in a static spacetime. The spatial
coordinate axes are fixed using a covariant Frenet triad so that the metric can
be expressed using the curvature and torsion of the spatial curve. As an
application of Fermi-Frenet coordinates, we show that they allow covariant
inertial forces to be expressed in a simple and physically intuitive way.Comment: 7 page
Optical frequency tripling with improved suppression and sideband selection
Journal Article, Impact factor:3.749A novel optical dispersion tolerant millimetre-wave radio-over-fibre system using optical frequency tripling technique with enhanced and selectable sideband suppression is demonstrated. The implementation utilises cascaded optical modulators to achieve either an optical single sideband (OSSB) or double sideband-suppressed carrier (DSB-SC) signal with high sideband suppression. Our analysis and simulation results indicate that the achievable suppression ratio of this configuration is only limited by other system factors such as optical noise and drifting of the operational conditions. The OSSB transmission system performance is assessed experimentally by the transport of 4 WiMax channels modulating a 10 GHz optical upconverted RF carrier as well as for optical frequency doubling and tripling. The 10 GHz and tripled carrier at 30 GHz are dispersion tolerant resulting both in an average relative constellation error (RCE) of -28.7 dB after 40 km of fibre. (C)2011 Optical Society of AmericaFundação para a Ciência e Tecnologi
Topological Properties from Einstein's Equations?
In this work we propose a new procedure for to extract global information of
a space-time. We considered a space-time immersed in a higher dimensional space
and we formulate the equations of Einstein through of the Frobenius conditions
to immersion. Through of an algorithm and the implementation into algebraic
computing system we calculate normal vectors from the immersion to find out the
second fundamental form. We make a application for space-time with spherical
symmetry and static. We solve the equations of Einstein to the vacuum and we
obtain space-times with different topologies.Comment: 7 pages, accepted for publication in Int. J. Mod. Phys.
Social inequalities: theories, concepts and problematics
This article aims to present a concise perusal of the different approaches developed in the study of social inequalities and in the relationships that they establish with manifold social processes and problems. The text does not intend to be exhaustive from the theoretical point of view, but rather to present an overview of the analytical complexity of the inequalities systems and demonstrate that they should be tackled in a multidimensional, systemic and multiscale perspective.info:eu-repo/semantics/publishedVersio
Embedding Versus Immersion in General Relativity
We briefly discuss the concepts of immersion and embedding of space-times in
higher-dimensional spaces. We revisit the classical work by Kasner in which he
constructs a model of immersion of the Schwarzschild exterior solution into a
six-dimensional pseudo-Euclidean manifold. We show that, from a physical point
of view, this model is not entirely satisfactory since the causal structure of
the immersed space-time is not preserved by the immersion.Comment: 5 page
- …