14,333 research outputs found
Diffusion of particles with short-range interactions
A system of interacting Brownian particles subject to short-range repulsive
potentials is considered. A continuum description in the form of a nonlinear
diffusion equation is derived systematically in the dilute limit using the
method of matched asymptotic expansions. Numerical simulations are performed to
compare the results of the model with those of the commonly used mean-field and
Kirkwood-superposition approximations, as well as with Monte Carlo simulation
of the stochastic particle system, for various interaction potentials. Our
approach works best for very repulsive short-range potentials, while the
mean-field approximation is suitable for long-range interactions. The Kirkwood
superposition approximation provides an accurate description for both short-
and long-range potentials, but is considerably more computationally intensive
Rediscounting Under Aggregate Risk with Moral Hazard
Freeman (1999) proposes a model in which discount window lending and open market operations have different effects. This is important because in most of the literature, these policies are indistinguishable. However, Freeman's argument that the central bank should absorb losses associated with default to provide risk-sharing stands in stark contrast to the concern that central banks should limit their exposure to credit risk. We extend Freeman's model by introducing moral hazard. With moral hazard, the central bank should avoid absorbing losses and Freeman's argument breaks down. However, we show that policies resembling discount window lending and open market operations can still be distinguished in this new framework. The optimal policy is for the central bank to make a restricted number of creditors compete for funds. By restricting the number of agents, the central bank can limit the moral hazard problem. By making them compete with each other, the central bank can exploit market information that reveals the state of the economy.Payment, clearing, and settlement systems; Financial markets; Central bank research
Greening critical care
Climate change and environmental stewardship are phrases that have been defining the past few decades and promoting change in our societies. The sensitivities of intensive care as a specialty make the process of greening an intensive care unit a challenge, but not one that is insurmountable. This paper discusses opportunities for critical care to reduce its environmental impact and provide a framework change. The article includes suggestions of what can be done as an individual, as a unit and as a hospital. Generally, practices in critical care are accepted without questioning the environmental consequences. We believe it is time for change, and critical care should give environmental stewardship a higher priority
A near zero velocity dispersion stellar component in the Canes Venatici dwarf spheroidal galaxy
We present a spectroscopic survey of the newly-discovered Canes Venatici
dwarf galaxy using the Keck/DEIMOS spectrograph. Two stellar populations of
distinct kinematics are found to be present in this galaxy: an extended,
metal-poor component, of half-light radius 7'.8(+2.4/-2.1), which has a
velocity dispersion of 13.9(+3.2/-2.5) km/s, and a more concentrated
(half-light radius 3'.6(+1.1/-0.8) metal-rich component of extremely low
velocity dispersion. At 99% confidence, the upper limit to the central velocity
dispersion of the metal-rich population is 1.9 km/s. This is the lowest
velocity dispersion ever measured in a galaxy. We perform a Jeans analysis on
the two components, and find that the dynamics of the structures can only be
consistent if we adopt extreme (and unlikely) values for the scale length and
velocity dispersion of the metal-poor population. With a larger radial velocity
sample and improved measurements of the density profile of the two populations,
we anticipate that it will be possible to place strong constraints on the
central distribution of the dark matter in this galaxy.Comment: 5 pages, 7 figures, accepted by MNRA
Algebraic and combinatorial aspects of sandpile monoids on directed graphs
The sandpile group of a graph is a well-studied object that combines ideas
from algebraic graph theory, group theory, dynamical systems, and statistical
physics. A graph's sandpile group is part of a larger algebraic structure on
the graph, known as its sandpile monoid. Most of the work on sandpiles so far
has focused on the sandpile group rather than the sandpile monoid of a graph,
and has also assumed the underlying graph to be undirected. A notable exception
is the recent work of Babai and Toumpakari, which builds up the theory of
sandpile monoids on directed graphs from scratch and provides many connections
between the combinatorics of a graph and the algebraic aspects of its sandpile
monoid.
In this paper we primarily consider sandpile monoids on directed graphs, and
we extend the existing theory in four main ways. First, we give a combinatorial
classification of the maximal subgroups of a sandpile monoid on a directed
graph in terms of the sandpile groups of certain easily-identifiable subgraphs.
Second, we point out certain sandpile results for undirected graphs that are
really results for sandpile monoids on directed graphs that contain exactly two
idempotents. Third, we give a new algebraic constraint that sandpile monoids
must satisfy and exhibit two infinite families of monoids that cannot be
realized as sandpile monoids on any graph. Finally, we give an explicit
combinatorial description of the sandpile group identity for every graph in a
family of directed graphs which generalizes the family of (undirected)
distance-regular graphs. This family includes many other graphs of interest,
including iterated wheels, regular trees, and regular tournaments.Comment: v2: Cleaner presentation, new results in final section. Accepted for
publication in J. Combin. Theory Ser. A. 21 pages, 5 figure
Cross-representational interactions: Interface and overlap mechanisms
A crucial question facing cognitive science concerns the nature of conceptual representations as well as the constraints on the interactions between them. One specific question we address in this paper is what makes cross representational interplay possible? We offer two distinct theoretical scenarios: According to the first scenario, co-activated knowledge representations interact with the help of an interface established between them via congruent activation in a mediating third party general cognitive mechanism, e.g., attention. According to the second scenario, co-activated knowledge representations interact due to an overlap between their features, for example when they share a magnitude component. First, we make a case for cross-representational interplay based on grounded and situated theories of cognition. Second, we discuss interface-based interactions between distinct (i.e., non-overlapping) knowledge representations. Third, we discuss how co-activated representations may share their architecture via partial overlap. Finally, we outline constraints regarding the flexibility of these proposed mechanisms
Differences in gregarine parasite load between male and female Calopteryx maculata
Natural History and EvolutionDamselflies and other insects of the order Odonata are frequently parasitized by gregarine protists. In the trophozoite stage of the gregarine life cycle, the parasite feeds on the contents of the hostâs gut and negatively affect its reproductive success. Possibly as a result of its impacts on the hostâs reproductive system, levels of gregarine parasitism has been observed to differ between male and female damselflies. We aimed to measure relative levels of gregarine parasitism of male and female damselflies of the species Calopteryx maculata . In order to do this, we collected damselflies at multiple sites in Michiganâs Maple River and dissected individuals to observe the presence of gregarine parasites. Our results indicate that females experience significantly higher levels of gregarine parasitism than their male counterparts. We propose that this difference is the result of increased levels of migratory behavior in female damselflies due to increased parental investment.https://deepblue.lib.umich.edu/bitstream/2027.42/147919/1/Chapman_Noyd_Seres_2018.pd
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