15 research outputs found
Excitable human dynamics driven by extrinsic events in massive communities
Using empirical data from a social media site (Twitter) and on trading
volumes of financial securities, we analyze the correlated human activity in
massive social organizations. The activity, typically excited by real-world
events and measured by the occurrence rate of international brand names and
trading volumes, is characterized by intermittent fluctuations with bursts of
high activity separated by quiescent periods. These fluctuations are broadly
distributed with an inverse cubic tail and have long-range temporal
correlations with a power spectrum. We describe the activity by a
stochastic point process and derive the distribution of activity levels from
the corresponding stochastic differential equation. The distribution and the
corresponding power spectrum are fully consistent with the empirical
observations.Comment: 9 pages, 3 figure
On the Anomalous Scaling Exponents in Nonlinear Models of Turbulence
We propose a new approach to the old-standing problem of the anomaly of the
scaling exponents of nonlinear models of turbulence. We achieve this by
constructing, for any given nonlinear model, a linear model of passive
advection of an auxiliary field whose anomalous scaling exponents are the same
as the scaling exponents of the nonlinear problem. The statistics of the
auxiliary linear model are dominated by `Statistically Preserved Structures'
which are associated with exact conservation laws. The latter can be used for
example to determine the value of the anomalous scaling exponent of the second
order structure function. The approach is equally applicable to shell models
and to the Navier-Stokes equations.Comment: revised version with new data on Navier-Stokes eq
Automatic Synonym Discovery with Knowledge Bases
Recognizing entity synonyms from text has become a crucial task in many
entity-leveraging applications. However, discovering entity synonyms from
domain-specific text corpora (e.g., news articles, scientific papers) is rather
challenging. Current systems take an entity name string as input to find out
other names that are synonymous, ignoring the fact that often times a name
string can refer to multiple entities (e.g., "apple" could refer to both Apple
Inc and the fruit apple). Moreover, most existing methods require training data
manually created by domain experts to construct supervised-learning systems. In
this paper, we study the problem of automatic synonym discovery with knowledge
bases, that is, identifying synonyms for knowledge base entities in a given
domain-specific corpus. The manually-curated synonyms for each entity stored in
a knowledge base not only form a set of name strings to disambiguate the
meaning for each other, but also can serve as "distant" supervision to help
determine important features for the task. We propose a novel framework, called
DPE, to integrate two kinds of mutually-complementing signals for synonym
discovery, i.e., distributional features based on corpus-level statistics and
textual patterns based on local contexts. In particular, DPE jointly optimizes
the two kinds of signals in conjunction with distant supervision, so that they
can mutually enhance each other in the training stage. At the inference stage,
both signals will be utilized to discover synonyms for the given entities.
Experimental results prove the effectiveness of the proposed framework
Kolmogorov scaling from random force fields
We show that the classical Kolmogorov and Richardson scaling laws in fully
developed turbulence are consistent with a random Gaussian force field.
Numerical simulations of a shell model approximation to the Navier-Stokes
equations suggest that the fluctuations in the force (acceleration) field are
scale independent throughout the inertial regime. We conjecture that Lagrangian
statistics of the relative velocity in a turbulent flow is determined by the
typical force field, whereas the multiscaling is associated to extreme events
in the force field fluctuations.Comment: 4 pages, 4 figure
Stress-driven phase transformation and the roughening of solid-solid interfaces
The application of stress to multiphase solid-liquid systems often results in
morphological instabilities. Here we propose a solid-solid phase transformation
model for roughening instability in the interface between two porous materials
with different porosities under normal compression stresses. This instability
is triggered by a finite jump in the free energy density across the interface,
and it leads to the formation of finger-like structures aligned with the
principal direction of compaction. The model is proposed as an explanation for
the roughening of stylolites - irregular interfaces associated with the
compaction of sedimentary rocks that fluctuate about a plane perpendicular to
the principal direction of compaction.Comment: (4 pages, 4 figures
Stochastic attractors for shell phenomenological models of turbulence
Recently, it has been proposed that the Navier-Stokes equations and a
relevant linear advection model have the same long-time statistical properties,
in particular, they have the same scaling exponents of their structure
functions. This assertion has been investigate rigorously in the context of
certain nonlinear deterministic phenomenological shell model, the Sabra shell
model, of turbulence and its corresponding linear advection counterpart model.
This relationship has been established through a "homotopy-like" coefficient
which bridges continuously between the two systems. That is, for
one obtains the full nonlinear model, and the corresponding linear
advection model is achieved for . In this paper, we investigate the
validity of this assertion for certain stochastic phenomenological shell models
of turbulence driven by an additive noise. We prove the continuous dependence
of the solutions with respect to the parameter . Moreover, we show the
existence of a finite-dimensional random attractor for each value of
and establish the upper semicontinuity property of this random attractors, with
respect to the parameter . This property is proved by a pathwise
argument. Our study aims toward the development of basic results and techniques
that may contribute to the understanding of the relation between the long-time
statistical properties of the nonlinear and linear models