4,990 research outputs found
Population growth and persistence in a heterogeneous environment: the role of diffusion and advection
The spatio-temporal dynamics of a population present one of the most
fascinating aspects and challenges for ecological modelling. In this article we
review some simple mathematical models, based on one dimensional
reaction-diffusion-advection equations, for the growth of a population on a
heterogeneous habitat. Considering a number of models of increasing complexity
we investigate the often contrary roles of advection and diffusion for the
persistence of the population. When it is possible we demonstrate basic
mathematical techniques and give the critical conditions providing the survival
of a population, in simple systems and in more complex resource-consumer models
which describe the dynamics of phytoplankton in a water column.Comment: Introductory review of simple conceptual models. 45 pages, 15 figures
v2: minor change
Thermal Lensing Spectroscopy With Picosecond Pulse Trains and a New Dual Beam Configuration
In this communication, we wish to report on the use of synchronously mode-locked picosecond lasers in a pump-probe configuration for TL spectroscopy. The peak power for these picosecond lasers is very high and, of course, the fundamental of the dye laser (red beam) can be efficiently (~10%) doubled in frequency (U.V. beam) by second harmonic generation in nonlinear crystals. We use this generated U.V. beam as a probe to monitor the very weak absorption of the red beam. An arrangement [4] of the beams involving different waist positions for the pump and probe is used. This arrangement results in an enhancement of sensitivity (at least a factor of three to seven). Also, a different dependence of the signal on the cell position compared to the single beam method is obtained. Finally, we have obtained the Δν = 5 CH-stretching overtone absorption spectrum of liquid toluene with this method (see Figure 1). Suggestions are made regarding new applications of this picosecond pulse thermal lensing technique
The externalisation of migration control in the European Union: first steps towards the external dimension of the space of freedom, security and justice
The creation of an area of freedom, security
and justice is one of the most rapidly
developing aspects of European integration.
It this paper, we take a look at the foreign
policies involved in this process — aside
from the internal development of the European
Union, they concern a significant
number of third countries, including Russia.
In our view, the efforts to manage the flow
of migrants and asylum seekers constitute a
viable part of the external dimension within
the AFSJ policies. Much of this article is
based on the theoretical postulates introduced
by the scholars of the Paris School, a
school within the discipline of security studies
that conceptualized the connection between
migration, terrorism, asylum, crime
and ethnic clashes, and its role as a major
threat facing the European Union. Externalization
of this complex threat (that is,
externalization in relation to the European
Union) is thus seen as one of the key prerequisites
to advancement of migration
management activities beyond the EU (i. e.
externalization of migration management).
In this article, we analyze the role the EU
plays at the international scene and categorize
the actions it took to manage the influx
of migrants and asylum seekers from the
1980s until the time when supranational
administrative bodies were granted mandates
in the spheres of Justice and Home
Affairs (JHA) of the EU Member States. We
conclude that it was as early as the 1990-s
that the EU launched the policy which later
allowed to transfer part of its security concerns
to third countries
Measurement of light mesons at RHIC by the PHENIX experiment
The PHENIX experiment at RHIC has measured a variety of light neutral mesons
(, K, , , , ) via
multi-particle decay channels over a wide range of transverse momentum. A
review of the recent results on the production rates of light mesons in p+p and
their nuclear modification factors in d+Au, Cu+Cu and Au+Au collisions at
different energies is presented.Comment: 5 pages, 4 figures, talk given at Hard Probes 2008 conference in La
Toja, Spain. submitted to EPJ
Analysis of patterns formed by two-component diffusion limited aggregation
We consider diffusion limited aggregation of particles of two different
kinds. It is assumed that a particle of one kind may adhere only to another
particle of the same kind. The particles aggregate on a linear substrate which
consists of periodically or randomly placed particles of different kinds. We
analyze the influence of initial patterns on the structure of growing clusters.
It is shown that at small distances from the substrate, the cluster structures
repeat initial patterns. However, starting from a critical distance the initial
periodicity is abruptly lost, and the particle distribution tends to a random
one. An approach describing the evolution of the number of branches is
proposed. Our calculations show that the initial patter can be detected only at
the distance which is not larger than approximately one and a half of the
characteristic pattern size.Comment: Accepted for publication in Physical Review
Survival of interacting Brownian particles in crowded 1D environment
We investigate a diffusive motion of a system of interacting Brownian
particles in quasi-one-dimensional micropores. In particular, we consider a
semi-infinite 1D geometry with a partially absorbing boundary and the hard-core
inter-particle interaction. Due to the absorbing boundary the number of
particles in the pore gradually decreases. We present the exact analytical
solution of the problem. Our procedure merely requires the knowledge of the
corresponding single-particle problem. First, we calculate the simultaneous
probability density of having still a definite number of surviving
particles at definite coordinates. Focusing on an arbitrary tagged particle, we
derive the exact probability density of its coordinate. Secondly, we present a
complete probabilistic description of the emerging escape process. The survival
probabilities for the individual particles are calculated, the first and the
second moments of the exit times are discussed. Generally speaking, although
the original inter-particle interaction possesses a point-like character, it
induces entropic repulsive forces which, e.g., push the leftmost (rightmost)
particle towards (opposite) the absorbing boundary thereby accelerating
(decelerating) its escape. More importantly, as compared to the reference
problem for the non-interacting particles, the interaction changes the
dynamical exponents which characterize the long-time asymptotic dynamics.
Interesting new insights emerge after we interpret our model in terms of a)
diffusion of a single particle in a -dimensional space, and b) order
statistics defined on a system of independent, identically distributed
random variables
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