3,234 research outputs found
mRNA of bovine tissue inhibitor of metalloproteinase: Sequence and expression in bovine ovarian tissue
Nonequilibrium Stationary States and Phase Transitions in Directed Ising Models
We study the nonequilibrium properties of directed Ising models with non
conserved dynamics, in which each spin is influenced by only a subset of its
nearest neighbours. We treat the following models: (i) the one-dimensional
chain; (ii) the two-dimensional square lattice; (iii) the two-dimensional
triangular lattice; (iv) the three-dimensional cubic lattice. We raise and
answer the question: (a) Under what conditions is the stationary state
described by the equilibrium Boltzmann-Gibbs distribution? We show that for
models (i), (ii), and (iii), in which each spin "sees" only half of its
neighbours, there is a unique set of transition rates, namely with exponential
dependence in the local field, for which this is the case. For model (iv), we
find that any rates satisfying the constraints required for the stationary
measure to be Gibbsian should satisfy detailed balance, ruling out the
possibility of directed dynamics. We finally show that directed models on
lattices of coordination number with exponential rates cannot
accommodate a Gibbsian stationary state. We conjecture that this property
extends to any form of the rates. We are thus led to the conclusion that
directed models with Gibbsian stationary states only exist in dimension one and
two. We then raise the question: (b) Do directed Ising models, augmented by
Glauber dynamics, exhibit a phase transition to a ferromagnetic state? For the
models considered above, the answers are open problems, to the exception of the
simple cases (i) and (ii). For Cayley trees, where each spin sees only the
spins further from the root, we show that there is a phase transition provided
the branching ratio, , satisfies
TRAVOS: Trust and Reputation in the Context of Inaccurate Information Sources
In many dynamic open systems, agents have to interact with one another to achieve their goals. Here, agents may be self-interested, and when trusted to perform an action for another, may betray that trust by not performing the action as required. In addition, due to the size of such systems, agents will often interact with other agents with which they have little or no past experience. There is therefore a need to develop a model of trust and reputation that will ensure good interactions among software agents in large scale open systems. Against this background, we have developed TRAVOS (Trust and Reputation model for Agent-based Virtual OrganisationS) which models an agent's trust in an interaction partner. Specifically, trust is calculated using probability theory taking account of past interactions between agents, and when there is a lack of personal experience between agents, the model draws upon reputation information gathered from third parties. In this latter case, we pay particular attention to handling the possibility that reputation information may be inaccurate
Statistics of quantum transmission in one dimension with broad disorder
We study the statistics of quantum transmission through a one-dimensional
disordered system modelled by a sequence of independent scattering units. Each
unit is characterized by its length and by its action, which is proportional to
the logarithm of the transmission probability through this unit. Unit actions
and lengths are independent random variables, with a common distribution that
is either narrow or broad. This investigation is motivated by results on
disordered systems with non-stationary random potentials whose fluctuations
grow with distance.
In the statistical ensemble at fixed total sample length four phases can be
distinguished, according to the values of the indices characterizing the
distribution of the unit actions and lengths. The sample action, which is
proportional to the logarithm of the conductance across the sample, is found to
obey a fluctuating scaling law, and therefore to be non-self-averaging, in
three of the four phases. According to the values of the two above mentioned
indices, the sample action may typically grow less rapidly than linearly with
the sample length (underlocalization), more rapidly than linearly
(superlocalization), or linearly but with non-trivial sample-to-sample
fluctuations (fluctuating localization).Comment: 26 pages, 4 figures, 1 tabl
Enhanced quantum tunnelling induced by disorder
We reconsider the problem of the enhancement of tunnelling of a quantum
particle induced by disorder of a one-dimensional tunnel barrier of length ,
using two different approximate analytic solutions of the invariant imbedding
equations of wave propagation for weak disorder. The two solutions are
complementary for the detailed understanding of important aspects of numerical
results on disorder-enhanced tunnelling obtained recently by Kim et al. (Phys.
rev. B{\bf 77}, 024203 (2008)). In particular, we derive analytically the
scaled wavenumber -threshold where disorder-enhanced tunnelling of an
incident electron first occurs, as well as the rate of variation of the
transmittance in the limit of vanishing disorder. Both quantities are in good
agreement with the numerical results of Kim et al. Our non-perturbative
solution of the invariant imbedding equations allows us to show that the
disorder enhances both the mean conductance and the mean resistance of the
barrier.Comment: 10 page
Dynamics at the angle of repose: jamming, bistability, and collapse
When a sandpile relaxes under vibration, it is known that its measured angle
of repose is bistable in a range of values bounded by a material-dependent
maximal angle of stability; thus, at the same angle of repose, a sandpile can
be stationary or avalanching, depending on its history. In the nearly jammed
slow dynamical regime, sandpile collapse to a zero angle of repose can also
occur, as a rare event. We claim here that fluctuations of {\it dilatancy} (or
local density) are the key ingredient that can explain such varied phenomena.
In this work, we model the dynamics of the angle of repose and of the density
fluctuations, in the presence of external noise, by means of coupled stochastic
equations. Among other things, we are able to describe sandpile collapse in
terms of an activated process, where an effective temperature (related to the
density as well as to the external vibration intensity) competes against the
configurational barriers created by the density fluctuations.Comment: 15 pages, 1 figure. Minor changes and update
Competition and cooperation:aspects of dynamics in sandpiles
In this article, we review some of our approaches to granular dynamics, now
well known to consist of both fast and slow relaxational processes. In the
first case, grains typically compete with each other, while in the second, they
cooperate. A typical result of {\it cooperation} is the formation of stable
bridges, signatures of spatiotemporal inhomogeneities; we review their
geometrical characteristics and compare theoretical results with those of
independent simulations. {\it Cooperative} excitations due to local density
fluctuations are also responsible for relaxation at the angle of repose; the
{\it competition} between these fluctuations and external driving forces, can,
on the other hand, result in a (rare) collapse of the sandpile to the
horizontal. Both these features are present in a theory reviewed here. An arena
where the effects of cooperation versus competition are felt most keenly is
granular compaction; we review here a random graph model, where three-spin
interactions are used to model compaction under tapping. The compaction curve
shows distinct regions where 'fast' and 'slow' dynamics apply, separated by
what we have called the {\it single-particle relaxation threshold}. In the
final section of this paper, we explore the effect of shape -- jagged vs.
regular -- on the compaction of packings near their jamming limit. One of our
major results is an entropic landscape that, while microscopically rough,
manifests {\it Edwards' flatness} at a macroscopic level. Another major result
is that of surface intermittency under low-intensity shaking.Comment: 36 pages, 23 figures, minor correction
The ART of IAM: The Winning Strategy for the 2006 Competition
In many dynamic open systems, agents have to interact with one another to achieve their goals. Here, agents may be self-interested, and when trusted to perform an action for others, may betray that trust by not performing the actions as required. In addition, due to the size of such systems, agents will often interact with other agents with which they have little or no past experience. This situation has led to the development of a number of trust and reputation models, which aim to facilitate an agent's decision making in the face of uncertainty regarding the behaviour of its peers. However, these multifarious models employ a variety of different representations of trust between agents, and measure performance in many different ways. This has made it hard to adequately evaluate the relative properties of different models, raising the need for a common platform on which to compare competing mechanisms. To this end, the ART Testbed Competition has been proposed, in which agents using different trust models compete against each other to provide services in an open marketplace. In this paper, we present the winning strategy for this competition in 2006, provide an analysis of the factors that led to this success, and discuss lessons learnt from the competition about issues of trust in multiagent systems in general. Our strategy, IAM, is Intelligent (using statistical models for opponent modelling), Abstemious (spending its money parsimoniously based on its trust model) and Moral (providing fair and honest feedback to those that request it)
Surface Properties of Aperiodic Ising Quantum Chains
We consider Ising quantum chains with quenched aperiodic disorder of the
coupling constants given through general substitution rules. The critical
scaling behaviour of several bulk and surface quantities is obtained by exact
real space renormalization.Comment: 4 pages, RevTex, reference update
- …