16,305 research outputs found
Model-based estimation of off-highway road geometry using single-axis LADAR and inertial sensing
This paper applies some previously studied extended
Kalman filter techniques for planar road geometry estimation
to the domain of autonomous navigation of off-highway
vehicles. In this work, a clothoid model of the road geometry is
constructed and estimated recursively based on road features
extracted from single-axis LADAR range measurements. We
present a method for feature extraction of the road centerline
in the image plane, and describe its application to recursive
estimation of the road geometry. We analyze the performance of
our method against simulated motion of varied road geometries
and against closed-loop detection, tracking and following of
desert roads. Our method accomodates full 6 DOF motion of
the vehicle as it navigates, constructs consistent estimates of the
road geometry with respect to a fixed global reference frame,
and requires an estimate of the sensor pose for each range
measurement
Threshold games and cooperation on multiplayer graphs
Objective: The study investigates the effect on cooperation in multiplayer
games, when the population from which all individuals are drawn is structured -
i.e. when a given individual is only competing with a small subset of the
entire population.
Method: To optimize the focus on multiplayer effects, a class of games were
chosen for which the payoff depends nonlinearly on the number of cooperators -
this ensures that the game cannot be represented as a sum of pair-wise
interactions, and increases the likelihood of observing behaviour different
from that seen in two-player games. The chosen class of games are named
"threshold games", and are defined by a threshold, , which describes the
minimal number of cooperators in a given match required for all the
participants to receive a benefit. The model was studied primarily through
numerical simulations of large populations of individuals, each with
interaction neighbourhoods described by various classes of networks.
Results: When comparing the level of cooperation in a structured population
to the mean-field model, we find that most types of structure lead to a
decrease in cooperation. This is both interesting and novel, simply due to the
generality and breadth of relevance of the model - it is likely that any model
with similar payoff structure exhibits related behaviour.
More importantly, we find that the details of the behaviour depends to a
large extent on the size of the immediate neighbourhoods of the individuals, as
dictated by the network structure. In effect, the players behave as if they are
part of a much smaller, fully mixed, population, which we suggest an expression
for.Comment: in PLOS ONE, 4th Feb 201
Vertical distribution and composition of phytoplankton under the influence of an upper mixed layer
The vertical distribution of phytoplankton is of fundamental importance for
the dynamics and structure of aquatic communities. Here, using an
advection-reaction-diffusion model, we investigate the distribution and
competition of phytoplankton species in a water column, in which inverse
resource gradients of light and a nutrient can limit growth of the biomass.
This problem poses a challenge for ecologists, as the location of a production
layer is not fixed, but rather depends on many internal parameters and
environmental factors. In particular, we study the influence of an upper mixed
layer (UML) in this system and show that it leads to a variety of dynamic
effects: (i) Our model predicts alternative density profiles with a maximum of
biomass either within or below the UML, thereby the system may be bistable or
the relaxation from an unstable state may require a long-lasting transition.
(ii) Reduced mixing in the deep layer can induce oscillations of the biomass;
we show that a UML can sustain these oscillations even if the diffusivity is
less than the critical mixing for a sinking phytoplankton population. (iii) A
UML can strongly modify the outcome of competition between different
phytoplankton species, yielding bistability both in the spatial distribution
and in the species composition. (iv) A light limited species can obtain a
competitive advantage if the diffusivity in the deep layers is reduced below a
critical value. This yields a subtle competitive exclusion effect, where the
oscillatory states in the deep layers are displaced by steady solutions in the
UML. Finally, we present a novel graphical approach for deducing the
competition outcome and for the analysis of the role of a UML in aquatic
systems.Comment: 20 pages, 8 figure
Transport- reaction modeling of marine gas hydrate deposits- global results
We have developed a multi-1D numerical model of gas hydrate formation and dissolution processes in anoxic marine sediments and, by this model, we have estimated the new global gas hydrate inventory (BURWICZ E. B. et al., 2011). The reaction-transport model contains various chemical compounds (solid organic carbon, dissolved methane, inorganic carbon, and sulfates, gas hydrates, and free methane gas). The rates of POC degradation, anaerobic methane oxidation, sulfate reduction, and methanogenesis are kinetically controlled. Gas hydrate stability zone (GHSZ) is defined as a combination of pressure, temperature, and (to a smaller degree) salinity conditions. The lower boundary of the GHSZ is defined as the intersection of gas hydrate and methane gas solubilities.
The diffusion equations are solved using a fully-implicit finite-differences method, while all transport processes are resolved by a Semi-Lagrangian scheme. Global input data sets (1°x1° resolution) were compiled from various oceanographic, geological and geophysical sources. The entire model was implemented in Matlab
Childhood Peer Status and the Clustering of Adverse Living Conditions in Adulthood
Within the context of the school class, children attain a social position in the peer hierarchy to which varying amounts of status are attached. Several studies have shown that childrenâs peer status is associated with a wide range of social and health-related outcomes. These studies commonly target separate outcomes, paying little attention to the fact that such circumstances are likely to go hand in hand. The overarching aim of the present study was therefore to examine the impact of childhood peer status on the clustering of living conditions in adulthood. Based on a 1953 cohort born in Stockholm, Sweden, multinomial regression analysis demonstrated that children who had lower peer status also had exceedingly high risks of ending up in more problem-burdened clusters as adults. Moreover, these associations remained after adjusting for a variety of family-related circumstances. We conclude that peer status constitutes a central aspect of childrenâs upbringing with important consequences for subsequent life chances, over and above the influences originating from the family.childhood; peer status; cohort; life course; outcome profiles; living conditions
Laser-induced bound-state phases in high-order harmonic generation
We present single-molecule and macroscopic calculations showing that
laser-induced Stark shifts contribute significantly to the phase of high-order
harmonics from polar molecules. This is important for orbital tomography, where
phases of field-free dipole matrix elements are needed in order to reconstruct
molecular orbitals. We derive an analytical expression that allows the
first-order Stark phase to be subtracted from experimental measurements
Maximal supersymmetry and exceptional groups
The article is a tribute to my old mentor, collaborator and friend Murray
Gell-Mann. In it I describe work by Pierre Ramond, Sung-Soo Kim and myself
where we describe the N = 8 Supergravity in the light-cone formalism. We show
how the Cremmer-Julia E7(7) non-linear symmetry is implemented and how the full
supermultiplet is a representation of the E7(7) symmetry. I also show how the
E7(7) symmetry is a key to understand the higher order couplings in the theory
and is very useful when we discuss possible counterterms for this theory.Comment: Proceedings of Conference in Honour of Murray Gell-Mann's 80th
Birthda
The First Supernova Explosions in the Universe
We investigate the supernova explosions that end the lives of massive
Population III stars in low-mass minihalos (M~10^6 M_sun) at redshifts z~20.
Employing the smoothed particle hydrodynamics method, we carry out numerical
simulations in a cosmological set-up of pair-instability supernovae with
explosion energies of E_SN=10^51 and 10^53 ergs. We find that the more
energetic explosion leads to the complete disruption of the gas in the
minihalo, whereas the lower explosion energy leaves much of the halo intact.
The higher energy supernova expels > 90% of the stellar metals into a region ~1
kpc across over a timescale of 3-5 Myr. Due to this burst-like initial star
formation episode, a large fraction of the universe could have been endowed
with a metallicity floor, Z_min>10^-4 Z_sun, already at z>15.Comment: Published in ApJ Letter
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